Add Transmission/NZBGet and per-client paths and much more

Extend multi-download-client support to include Transmission and NZBGet and introduce per-client custom download paths. Adds protocol mapping and new client types, Transmission/NZBGet integration services, API CRUD and validation changes, UI components/modal updates and live path previews, and manager routing by protocol. Includes DB migrations (download_path on download_history, interactive_search_access on users), schema updates, and related processor/service fixes and tests to ensure backward compatibility and proper path resolution.

documentation/phase3/README.md

@@ -15,7 +15,7 @@ Request → search_indexers → rank_results → download_torrent
 
 1. **search_indexers** - Search Prowlarr for torrents
 2. **rank_results** - Apply ranking algorithm, select best
-3. **download_torrent** - Add to qBittorrent
+3. **download_torrent** - Add to download client (qBittorrent/Transmission/SABnzbd)
 4. **monitor_download** - Poll progress (10s intervals)
 5. **process_audiobook** - Organize files to media directory
 6. **update_plex** - Trigger scan, fuzzy match
@@ -23,7 +23,7 @@ Request → search_indexers → rank_results → download_torrent
 ## Integration Points
 
 **Indexers:** Prowlarr (primary), Jackett (fallback)
-**Download Clients:** qBittorrent (primary), Transmission (fallback)
+**Download Clients:** qBittorrent or Transmission (torrent), SABnzbd (usenet) — [details](./download-clients.md)
 **Media Server:** Plex (scan + match)
 
 ## Job Queue (Bull)
@@ -43,7 +43,9 @@ Request → search_indexers → rank_results → download_torrent
 ## Related Docs
 
 - [Prowlarr](./prowlarr.md)
+- [Download Clients](./download-clients.md) - Multi-client management, protocol routing
 - [qBittorrent](./qbittorrent.md)
+- [SABnzbd](./sabnzbd.md)
 - [Ranking Algorithm](./ranking-algorithm.md)
 - [File Organization](./file-organization.md)
 - [Plex Integration](../integrations/plex.md)

documentation/phase3/download-clients.md

@@ -1,45 +1,127 @@
 # Multi-Download-Client Support
 
-**Status:** ✅ Implemented | Simultaneous qBittorrent + SABnzbd support
+**Status:** ✅ Implemented | qBittorrent, Transmission, SABnzbd, and NZBGet support
 
 ## Overview
-Users can configure both qBittorrent (torrents) and SABnzbd (Usenet) simultaneously. System selects best release across all indexer types regardless of protocol.
+Users can configure one torrent client (qBittorrent or Transmission) and one usenet client (SABnzbd or NZBGet) simultaneously. System selects best release across all indexer types regardless of protocol.
 
-**Constraint:** 1 client per type (torrent/usenet) for now; architecture supports future expansion.
+**Constraint:** 1 client per protocol (torrent/usenet). Users must remove an existing torrent client before adding a different one.
 
 ## Key Details
 
+### Supported Clients
+
+| Client | Protocol | Auth | Categories |
+|--------|----------|------|------------|
+| qBittorrent | torrent | Cookie-based (login endpoint) | Categories |
+| Transmission | torrent | HTTP Basic Auth + CSRF (`X-Transmission-Session-Id`) | Labels |
+| SABnzbd | usenet | API key | Categories |
+| NZBGet | usenet | HTTP Basic Auth (JSON-RPC) | Config-based categories |
+
+### Protocol Map
+**File:** `src/lib/interfaces/download-client.interface.ts`
+
+```typescript
+export const CLIENT_PROTOCOL_MAP: Record<DownloadClientType, ProtocolType> = {
+  qbittorrent: 'torrent',
+  sabnzbd: 'usenet',
+  nzbget: 'usenet',
+  transmission: 'torrent',
+};
+```
+
+Used by manager's `getClientForProtocol()` and UI's protocol-level enforcement.
+
 ### Configuration Structure
 **Key:** `download_clients` (JSON array, replaces legacy flat keys)
 
 ```typescript
 interface DownloadClientConfig {
   id: string;                    // UUID
-  type: 'qbittorrent' | 'sabnzbd';
+  type: 'qbittorrent' | 'sabnzbd' | 'nzbget' | 'transmission';
   name: string;                  // User-friendly name
   enabled: boolean;
   url: string;
-  username?: string;             // qBittorrent only
+  username?: string;             // qBittorrent/Transmission/NZBGet only
   password: string;              // Password or API key
   disableSSLVerify: boolean;
   remotePathMappingEnabled: boolean;
   remotePath?: string;
   localPath?: string;
   category?: string;             // Default: 'readmeabook'
+  customPath?: string;           // Relative sub-path appended to download_dir
 }
 ```
 
+### Transmission Service
+**File:** `src/lib/integrations/transmission.service.ts`
+
+- **RPC endpoint:** `POST /transmission/rpc` (JSON-RPC)
+- **CSRF:** 409 → capture `X-Transmission-Session-Id` header → retry
+- **Auth:** HTTP Basic Auth (optional)
+- **Categories:** Uses `labels` array on `torrent-add`
+- **Download path:** `download-dir` argument on `torrent-add`
+- **Torrent files:** Base64-encoded via `metainfo` field
+- **Status codes:** 0=stopped→paused, 1=check-pending→checking, 2=checking→checking, 3=download-pending→queued, 4=downloading→downloading, 5=seed-pending→seeding, 6=seeding→seeding
+- **Error handling:** `error > 0` → failed status
+- **postProcess():** No-op (same as qBittorrent)
+
+### NZBGet Service
+**File:** `src/lib/integrations/nzbget.service.ts`
+
+- **RPC endpoint:** `POST /jsonrpc` (JSON-RPC with Basic Auth)
+- **Auth:** HTTP Basic Auth (username + password)
+- **Categories:** Config-based (`Category1.Name`, `Category1.DestDir`), managed via `config()` + `saveconfig()`
+- **Adding NZBs:** Downloads NZB content from Prowlarr, base64-encodes, uploads via `append()`
+- **Queue status:** `listgroups(0)` — QUEUED, PAUSED, DOWNLOADING, FETCHING, PP_* (processing states)
+- **History status:** `history(false)` — SUCCESS/*, WARNING/* → completed; FAILURE/*, DELETED/* → failed
+- **Pause/Resume/Delete:** `editqueue()` with GroupPause/GroupResume/GroupDelete/HistoryDelete commands
+- **postProcess():** `editqueue('HistoryDelete')` — archives from visible history (preserves in hidden archive)
+- **IDs:** Integer NZBIDs (stored as strings in RMAB system)
+
+### Per-Client Custom Download Path
+**Field:** `customPath` (optional string, blank = use base `download_dir` as-is)
+
+Allows each download client to download to a different subdirectory under `download_dir`. Useful for separating torrent and usenet downloads.
+
+**Path Resolution (in `createService()`):**
+```
+finalPath = config.customPath ? path.join(downloadDir, config.customPath) : downloadDir
+```
+
+**Example:**
+- `download_dir` = `/downloads`, qBittorrent `customPath` = `torrents` → `/downloads/torrents`
+- `download_dir` = `/downloads`, SABnzbd `customPath` = `usenet` → `/downloads/usenet`
+- `download_dir` = `/downloads`, `customPath` = blank → `/downloads`
+
+**Validation:**
+- Leading/trailing slashes stripped on save
+- Paths containing `..` rejected (frontend + API)
+- Backward-compatible: existing configs without `customPath` default to base `download_dir`
+
+**Resolved path used by:**
+- Service constructors (`defaultSavePath` / `defaultDownloadDir`)
+- Category creation (qBittorrent `ensureCategory`, SABnzbd `ensureCategory`)
+- Torrent/NZB addition (save path / download-dir)
+- Remote path mapping (applied after customPath resolution)
+- Singleton getters (`getQBittorrentService`, `getSABnzbdService`)
+- Retry fallback path construction (`retry-failed-imports.processor.ts`)
+
+**UI:** Modal shows real-time path preview: `Downloads to: /downloads/torrents`
+
 ### Download Client Manager Service
 **File:** `src/lib/services/download-client-manager.service.ts`
 
 **Methods:**
-- `getClientForProtocol(protocol: 'torrent' | 'usenet')` - Get client by protocol
+- `getClientForProtocol(protocol: 'torrent' | 'usenet')` - Get client by protocol (uses `CLIENT_PROTOCOL_MAP`)
 - `hasClientForProtocol(protocol)` - Check if protocol configured
 - `getAllClients()` - List all configs
 - `testConnection(config)` - Test specific config
 - `invalidate()` - Clear cache on config change
 - `getClientServiceForProtocol(protocol)` - Get instantiated service
 
+**Factory Cases:** `qbittorrent` → `QBittorrentService`, `sabnzbd` → `SABnzbdService`, `nzbget` → `NZBGetService`, `transmission` → `TransmissionService`
+
 **Singleton Pattern:** Uses caching with invalidation on config changes.
 
 ### Protocol Filtering
@@ -57,7 +139,7 @@ interface DownloadClientConfig {
 **Logic:**
 1. Detect protocol from result (`ProwlarrService.isNZBResult()`)
 2. Get appropriate client via manager (`getClientForProtocol()`)
-3. Route to qBittorrent or SABnzbd service
+3. Route to correct service (qBittorrent, Transmission, or SABnzbd)
 4. Create download history record
 
 ### Migration
@@ -76,7 +158,7 @@ interface DownloadClientConfig {
 **POST /api/admin/settings/download-clients/test** - Test connection
 
 **Validation:**
-- Only 1 client per type allowed (enforced on add)
+- Only 1 client per protocol allowed (enforced on add via `CLIENT_PROTOCOL_MAP`)
 - Test connection required before save
 - Password masking in responses (`********`)
 
@@ -86,14 +168,18 @@ interface DownloadClientConfig {
 
 | Component | Purpose |
 |-----------|---------|
-| `DownloadClientManagement.tsx` | Container with add buttons + configured cards |
-| `DownloadClientCard.tsx` | Card with name, type badge, edit/delete |
-| `DownloadClientModal.tsx` | Add/edit modal with type-specific fields |
+| `DownloadClientManagement.tsx` | Container with add cards (4-column: qBittorrent, Transmission, SABnzbd, NZBGet) + configured cards; protocol-level enforcement (grayed out when protocol taken) |
+| `DownloadClientCard.tsx` | Card with name, type badge (blue=qBittorrent, green=Transmission, purple=SABnzbd, orange=NZBGet), custom path display, edit/delete |
+| `DownloadClientModal.tsx` | Add/edit modal with type-specific fields; Username shown for qBittorrent + Transmission + NZBGet; URL placeholder per-type |
 
 **UI Flow:**
-1. **Add Client Section:** Two cards (qBittorrent, SABnzbd) with "Add" button or "Already configured" badge
-2. **Configured Clients:** Grid of cards showing name, type, URL, status
-3. **Modal:** Type-specific fields, SSL toggle, path mapping, test connection
+1. **Add Client Section:** Four cards (qBittorrent, Transmission, SABnzbd, NZBGet) with "Add" button or "Protocol already configured" when protocol is taken (card grayed out with `opacity-50`)
+2. **Configured Clients:** Grid of cards showing name, type, URL, custom path (if set), status
+3. **Modal:** Type-specific fields, custom download path with live preview, SSL toggle, path mapping, test connection
+
+**downloadDir Prop Flow:**
+- **Settings mode:** `DownloadClientManagement` fetches from `GET /api/admin/settings` → `settings.paths.downloadDir` on mount
+- **Wizard mode:** `setup/page.tsx` passes `state.downloadDir` → `DownloadClientStep` → `DownloadClientManagement` → `DownloadClientModal`
 
 ## Integration Points
 
@@ -107,6 +193,8 @@ Replaced legacy form with `<DownloadClientManagement mode="settings" />`
 
 Replaced single-client form with `<DownloadClientManagement mode="wizard" />`
 
+**Props:** Accepts `downloadDir` from setup page state, passes to management component
+
 **Validation:** At least 1 enabled client required to proceed
 
 ### Setup Complete API
@@ -123,25 +211,38 @@ Accepts both legacy single client and new array format:
 **Client disabled:** Results for that protocol filtered out
 **Connection failure:** Per-download error handling (existing)
 **Mixed results:** Best release selected regardless of protocol when both clients configured
+**Custom path blank:** Uses base `download_dir` (backward-compatible default)
+**Custom path with slashes:** Leading/trailing slashes stripped automatically
+**Custom path with `..`:** Rejected by frontend validation and API validation
+**Switching torrent clients:** Must delete existing torrent client before adding Transmission (or vice versa)
 
 ## Verification Steps
 
 1. **Migration:** Existing single-client users see config as card after update
 2. **Single client:** Configure only qBittorrent → only torrent results shown
-3. **Both clients:** Configure both → mixed results, best selected across protocols
-4. **Download routing:** Torrent result → qBittorrent; NZB result → SABnzbd
+3. **Both clients:** Configure torrent + usenet → mixed results, best selected across protocols
+4. **Download routing:** Torrent result → torrent client; NZB result → usenet client (SABnzbd or NZBGet)
 5. **Wizard:** Must add at least one client to proceed
 6. **Settings:** Can add/edit/delete/test clients; changes persist
+7. **Custom path:** Set `torrents` on torrent client → save path includes subdirectory
+8. **Custom path preview:** Modal shows resolved path in real-time as user types
+9. **Custom path persistence:** Save, reopen modal → value persists
+10. **Custom path on card:** Configured cards show custom path if set
+11. **Transmission CSRF:** First RPC call gets 409, captures session ID, retry succeeds
+12. **Protocol enforcement:** Adding qBittorrent grays out Transmission card (and vice versa)
 
 ## Critical Files
 
 | File | Changes |
 |------|---------|
-| `src/lib/services/download-client-manager.service.ts` | **NEW** - Core multi-client service |
+| `src/lib/interfaces/download-client.interface.ts` | Client types, display names, `CLIENT_PROTOCOL_MAP` |
+| `src/lib/integrations/nzbget.service.ts` | NZBGet JSON-RPC implementation |
+| `src/lib/integrations/transmission.service.ts` | Transmission RPC implementation |
+| `src/lib/services/download-client-manager.service.ts` | Core multi-client service, protocol-based routing |
 | `src/lib/integrations/prowlarr.service.ts:379` | Protocol filtering logic (both clients = all results) |
 | `src/lib/processors/download-torrent.processor.ts:44` | Download routing (detect protocol → route) |
-| `src/app/api/admin/settings/download-clients/*` | **NEW** - CRUD API routes |
-| `src/components/admin/download-clients/*` | **NEW** - UI components (card-based) |
+| `src/app/api/admin/settings/download-clients/*` | CRUD API routes, protocol-level duplicate check |
+| `src/components/admin/download-clients/*` | UI components (3-column card layout, protocol enforcement) |
 | `src/app/admin/settings/tabs/DownloadTab/DownloadTab.tsx` | Replaced with management component |
 | `src/app/setup/steps/DownloadClientStep.tsx` | Replaced with management component |
 | `src/app/api/setup/complete/route.ts` | Save as JSON array, support legacy |
@@ -149,5 +250,5 @@ Accepts both legacy single client and new array format:
 ## Related
 
 - [qBittorrent Integration](./qbittorrent.md) - Torrent client details
-- [SABnzbd Integration](./sabnzbd.md) - Usenet client details
+- [SABnzbd Integration](./sabnzbd.md) - Usenet client details (SABnzbd)
 - [Prowlarr Integration](./prowlarr.md) - Indexer search

documentation/phase3/file-organization.md

@@ -37,7 +37,8 @@ Result: Douglas Adams/Stephen Fry/The Hitchhiker's Guide to the Galaxy/
 ## Process
 
 1. Download completes in `/downloads/[torrent-name]/` or `/downloads/[filename]` (single file)
-2. Identify audiobook files (.m4b, .m4a, .mp3) - supports both directories and single files
+1b. **Path stored** in `DownloadHistory.downloadPath` (mapped local path) for retry reliability — avoids reconstructing path from `torrentName` which may differ from actual folder name
+2. Identify audiobook files (.m4b, .m4a, .mp3, .mp4, .aa, .aax, .flac, .ogg) - supports both directories and single files
 3. Read media directory and path template from database config (`media_dir`, `audiobook_path_template`)
 4. Apply template to create target path: `[media_dir]/[template result]/`
 5. **Copy** files (not move - originals stay for seeding)
@@ -94,6 +95,7 @@ Result: Douglas Adams/Stephen Fry/The Hitchhiker's Guide to the Galaxy/
 **Supported Formats:**
 - m4b, m4a, mp4 (AAC audiobooks)
 - mp3 (ID3v2 tags)
+- flac (Vorbis comment tags)
 
 **Metadata Written:**
 - `title` - Book title

documentation/phase3/qbittorrent.md

@@ -46,7 +46,7 @@ Free, open-source BitTorrent client with comprehensive Web API.
 - `download_client_url` - qBittorrent Web UI URL (supports HTTP and HTTPS)
 - `download_client_username` - qBittorrent username
 - `download_client_password` - qBittorrent password
-- `download_dir` - Download save path (passed to qBittorrent for all torrents)
+- `download_dir` - Base download save path (joined with per-client `customPath` if configured)
 
 **Optional (SSL/TLS):**
 - `download_client_disable_ssl_verify` - Disable SSL certificate verification for HTTPS (boolean as string "true"/"false", default: "false")
@@ -65,7 +65,8 @@ Validation: All required fields checked before service initialization. Path mapp
 Service uses singleton pattern for performance. When settings change (via admin settings page), singleton is invalidated to force reload:
 - `invalidateQBittorrentService()` called after updating paths or download client settings
 - Forces service to re-read database config on next torrent addition
-- Ensures category save path and credentials are always current
+- Ensures category save path, credentials, and `customPath` resolution are always current
+- Singleton getter resolves `customPath` from client config (consistent with manager's `createService()`)
 
 ## Category Management
 
@@ -73,9 +74,10 @@ Service uses singleton pattern for performance. When settings change (via admin
 
 **Save Path Synchronization:**
 - Category created/updated on every torrent addition
-- Category save path always synced with `download_dir` config
-- Handles config changes: if user changes `download_dir`, category updates automatically
+- Category save path synced with resolved download path (`download_dir` + per-client `customPath`)
+- Handles config changes: if user changes `download_dir` or `customPath`, category updates automatically
 - Uses both `createCategory` and `editCategory` APIs for reliability
+- Remote path mapping applied after `customPath` resolution (outgoing: local → remote)
 
 **Why Both Create and Edit:**
 1. Create: Ensures category exists (idempotent, won't fail if exists)
@@ -83,6 +85,11 @@ Service uses singleton pattern for performance. When settings change (via admin
 
 This prevents issues where category retains old save path after user changes `download_dir` setting.
 
+**Per-Client Custom Path:**
+- If `customPath` is set (e.g., `torrents`), category save path becomes `/downloads/torrents`
+- Remote path mapping applies to the resolved path: `reverseTransform(/downloads/torrents)` → remote equivalent
+- See [download-clients.md](./download-clients.md#per-client-custom-download-path) for details
+
 ## Remote Path Mapping
 
 **Use Case:** qBittorrent runs on different machine/container with different filesystem perspective.
@@ -167,8 +174,22 @@ interface TorrentInfo {
   completionDate: number;
 }
 
-type TorrentState = 'downloading' | 'uploading' | 'stalledDL' |
-  'pausedDL' | 'queuedDL' | 'checkingDL' | 'error' | 'missingFiles';
+type TorrentState =
+  // Core states
+  | 'downloading' | 'uploading'
+  | 'stalledDL' | 'stalledUP'
+  | 'pausedDL' | 'pausedUP'
+  | 'queuedDL' | 'queuedUP'
+  | 'checkingDL' | 'checkingUP'
+  | 'error' | 'missingFiles' | 'allocating'
+  // Forced states (user clicked "Force Resume")
+  | 'forcedDL' | 'forcedUP'
+  // Metadata fetching
+  | 'metaDL' | 'forcedMetaDL'
+  // qBittorrent v5.0+ (renamed paused → stopped)
+  | 'stoppedDL' | 'stoppedUP'
+  // Other
+  | 'checkingResumeData' | 'moving';
 ```
 
 ## Fixed Issues ✅
@@ -216,6 +237,12 @@ type TorrentState = 'downloading' | 'uploading' | 'stalledDL' |
    - Service constructor accepts `PathMappingConfig` parameter
    - Singleton loads path mapping config from database
 
+**15. Missing qBittorrent torrent states** - Monitor never detected completion for force-resumed torrents (`forcedDL`/`forcedUP`), causing infinite polling at 100%. Also missing metadata states (`metaDL`/`forcedMetaDL`), qBittorrent v5.x renamed states (`stoppedDL`/`stoppedUP`), and utility states (`checkingResumeData`/`moving`). Fixed by:
+   - Adding all 8 missing states to `TorrentState` type union
+   - Adding mappings to both `mapState()` (legacy) and `mapStateToDownloadStatus()` (unified interface)
+   - `forcedUP` → `seeding`/`completed` enables monitor to trigger import
+   - `stoppedDL`/`stoppedUP` → `paused` ensures qBittorrent v5.x compatibility
+
 ## Tech Stack
 
 - axios (HTTP + cookie mgmt)

documentation/phase3/ranking-algorithm.md

@@ -135,12 +135,13 @@ Evaluates and scores torrents to automatically select best audiobook download.
 - Proportional credit: If 2 of 3 authors match → 10 pts (2/3 × 15)
 - Full credit: If all authors match → 15 pts
 
-**2. Format Quality (25 pts max)**
-- M4B with chapters: 25
-- M4B without chapters: 22
-- M4A: 16
-- MP3: 10
-- Other: 3
+**2. Format Quality (10 pts max)**
+- M4B with chapters: 10
+- M4B without chapters: 9
+- FLAC: 7 (lossless audio)
+- M4A: 6
+- MP3: 4
+- Other: 1
 
 **3. Seeder Count (15 pts max)**
 - Formula: `Math.min(15, Math.log10(seeders + 1) * 6)`

documentation/phase3/sabnzbd.md

@@ -19,7 +19,7 @@ Free, open-source Usenet/NZB download client with comprehensive Web API. Industr
 **Format:** All requests use `output=json` for JSON responses
 
 **GET /api?mode=version&output=json&apikey={key}** - Get SABnzbd version
-**GET /api?mode=addurl&name={url}&cat={category}&output=json&apikey={key}** - Add NZB by URL
+**POST /api (multipart: mode=addfile, nzbfile={binary})** - Add NZB by file upload (RMAB downloads NZB from Prowlarr, uploads to SABnzbd)
 **GET /api?mode=queue&output=json&apikey={key}** - Get active downloads
 **GET /api?mode=history&limit=100&output=json&apikey={key}** - Get completed/failed downloads
 **GET /api?mode=pause&value={nzbId}&output=json&apikey={key}** - Pause download
@@ -37,7 +37,7 @@ Free, open-source Usenet/NZB download client with comprehensive Web API. Industr
 - `download_client_type` - Must be 'sabnzbd'
 - `download_client_url` - SABnzbd Web UI URL (supports HTTP and HTTPS)
 - `download_client_password` - API key (reuses password field)
-- `download_dir` - Download save path (passed to SABnzbd category)
+- `download_dir` - Base download save path (joined with per-client `customPath` if configured)
 
 **Optional (SSL/TLS):**
 - `download_client_disable_ssl_verify` - Disable SSL certificate verification (boolean as string "true"/"false", default: "false")
@@ -58,7 +58,8 @@ Validation: All required fields checked before service initialization. Path mapp
 Service uses singleton pattern. When settings change, singleton invalidated to force reload:
 - `invalidateSABnzbdService()` called after updating settings
 - Forces service to re-read database config
-- Ensures category and credentials are always current
+- Ensures category, credentials, and `customPath` resolution are always current
+- Singleton getter resolves `customPath` from client config (consistent with manager's `createService()`)
 
 ## Category Management
 
@@ -67,16 +68,21 @@ Service uses singleton pattern. When settings change, singleton invalidated to f
 **Save Path Synchronization:**
 - Category created/updated on every download (matches qBittorrent behavior)
 - Fetches SABnzbd's `complete_dir` setting via API to understand download location
-- Applies remote path mapping to translate RMAB's `download_dir` to SABnzbd's perspective
+- Applies remote path mapping to translate RMAB's resolved download path to SABnzbd's perspective
 - Calculates optimal category path (relative, absolute, or root)
+- Resolved path includes per-client `customPath` if configured (e.g., `/downloads/usenet`)
 
 **Smart Path Calculation:**
 1. Get SABnzbd's `complete_dir` from `misc.complete_dir` config
-2. Apply `PathMapper.reverseTransform()` to RMAB's `download_dir`
+2. Apply `PathMapper.reverseTransform()` to RMAB's resolved download path (`download_dir` + `customPath`)
 3. Compare transformed path to `complete_dir`:
    - **Match:** Use empty string (downloads go to complete_dir root)
-   - **Subdirectory:** Use relative path (e.g., `audiobooks`)
-   - **Different:** Use absolute path (e.g., `/mnt/media/audiobooks`)
+   - **Subdirectory:** Use relative path (e.g., `usenet`)
+   - **Different:** Use absolute path (e.g., `/mnt/media/usenet`)
+
+**Per-Client Custom Path:**
+- If `customPath` is set (e.g., `usenet`), category path calculated from `/downloads/usenet`
+- See [download-clients.md](./download-clients.md#per-client-custom-download-path) for details
 
 ## Post-Processing
 
@@ -290,9 +296,20 @@ organizePath = PathMapper.transform(sabPath, config)
 | Path Mapping | ✅ Bidirectional (same as qBit) | ✅ Bidirectional |
 | Category Sync | ✅ Every download | ✅ Every download |
 
+## NZB Download Proxy
+
+**RMAB proxies NZB files** — SABnzbd does not need network access to Prowlarr.
+
+Prowlarr returns download URLs that point back to itself (proxy URLs like `http://prowlarr:9696/3/download?apikey=...&link=...`).
+RMAB downloads the NZB file content from that URL, then uploads it to SABnzbd via `mode=addfile` (multipart POST).
+This matches qBittorrent's pattern where RMAB downloads `.torrent` files and uploads the binary content.
+
+**Result:** Download clients only need network access to RMAB. No direct Prowlarr connectivity required.
+
 ## Tech Stack
 
-- axios (HTTP client)
+- axios (HTTP client, NZB file download)
+- form-data (multipart file upload to SABnzbd)
 - Node.js https (SSL/TLS agent)
 - JSON API responses