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/TABLEOFCONTENTS.md

@@ -52,7 +52,7 @@
 - **Full pipeline overview** → [phase3/README.md](phase3/README.md)
 - **Search via Prowlarr (torrents + NZBs)** → [phase3/prowlarr.md](phase3/prowlarr.md)
 - **Torrent ranking/selection** → [phase3/ranking-algorithm.md](phase3/ranking-algorithm.md)
-- **Multi-download-client support (qBittorrent + SABnzbd)** → [phase3/download-clients.md](phase3/download-clients.md)
+- **Multi-download-client support (qBittorrent, Transmission, SABnzbd, NZBGet)** → [phase3/download-clients.md](phase3/download-clients.md)
 - **qBittorrent integration (torrents)** → [phase3/qbittorrent.md](phase3/qbittorrent.md)
 - **SABnzbd integration (Usenet/NZB)** → [phase3/sabnzbd.md](phase3/sabnzbd.md)
 - **File organization, seeding** → [phase3/file-organization.md](phase3/file-organization.md)
@@ -111,8 +111,11 @@
 **"How do I add a new audiobook?"** → [integrations/audible.md](integrations/audible.md) (scraping), [phase3/README.md](phase3/README.md) (automation)
 **"How do I configure multiple download clients?"** → [phase3/download-clients.md](phase3/download-clients.md)
 **"How do torrent downloads work?"** → [phase3/qbittorrent.md](phase3/qbittorrent.md), [backend/services/jobs.md](backend/services/jobs.md)
-**"How do Usenet/NZB downloads work?"** → [phase3/sabnzbd.md](phase3/sabnzbd.md), [backend/services/jobs.md](backend/services/jobs.md)
+**"How do Usenet/NZB downloads work?"** → [phase3/sabnzbd.md](phase3/sabnzbd.md), [phase3/download-clients.md](phase3/download-clients.md), [backend/services/jobs.md](backend/services/jobs.md)
 **"Can I use both qBittorrent and SABnzbd?"** → [phase3/download-clients.md](phase3/download-clients.md)
+**"How do I use NZBGet instead of SABnzbd?"** → [phase3/download-clients.md](phase3/download-clients.md)
+**"How do I use Transmission instead of qBittorrent?"** → [phase3/download-clients.md](phase3/download-clients.md)
+**"How do I set different download paths per client?"** → [phase3/download-clients.md](phase3/download-clients.md#per-client-custom-download-path)
 **"How does Plex matching work?"** → [integrations/plex.md](integrations/plex.md)
 **"How does e-book support work?"** → [integrations/ebook-sidecar.md](integrations/ebook-sidecar.md)
 **"How do I enable e-book downloads?"** → [integrations/ebook-sidecar.md](integrations/ebook-sidecar.md), [settings-pages.md](settings-pages.md#e-book-sidecar)
@@ -134,7 +137,7 @@
 **"How do I deploy?"** → [deployment/docker.md](deployment/docker.md) (multi-container), [deployment/unified.md](deployment/unified.md) (all-in-one)
 **"How do I use the unified container?"** → [deployment/unified.md](deployment/unified.md)
 **"Why can't RMAB find my downloaded files?"** → [deployment/volume-mapping.md](deployment/volume-mapping.md)
-**"How do I set up volume mapping for qBittorrent/SABnzbd?"** → [deployment/volume-mapping.md](deployment/volume-mapping.md)
+**"How do I set up volume mapping for qBittorrent/Transmission/SABnzbd/NZBGet?"** → [deployment/volume-mapping.md](deployment/volume-mapping.md)
 **"OAuth redirects to localhost / PUBLIC_URL not working"** → [backend/services/environment.md](backend/services/environment.md)
 **"What environment variables do I need?"** → [backend/services/environment.md](backend/services/environment.md)
 **"How does chapter merging work?"** → [features/chapter-merging.md](features/chapter-merging.md)