Add SABnzbd Usenet/NZB integration and documentation

Introduces SABnzbd as a supported download client for Usenet/NZB alongside qBittorrent, including service implementation, setup wizard and admin settings UI updates, and protocol-specific job processor logic. Updates documentation, PRD, and database schema to support NZB downloads, adds comprehensive technical details and testing strategies, and fixes Audible integration issues related to search and ASIN extraction.

prisma/schema.prisma

@@ -238,6 +238,7 @@ model DownloadHistory {
   indexerName      String    @map("indexer_name")
   torrentName      String?   @map("torrent_name")
   torrentHash      String?   @map("torrent_hash")
+  nzbId            String?   @map("nzb_id") // SABnzbd NZB ID (mutually exclusive with torrentHash)
   torrentSizeBytes BigInt?   @map("torrent_size_bytes")
   magnetLink       String?   @map("magnet_link") @db.Text
   torrentUrl       String?   @map("torrent_url") @db.Text
@@ -245,7 +246,7 @@ model DownloadHistory {
   leechers         Int?
   qualityScore     Int?      @map("quality_score")
   selected         Boolean   @default(false)
-  downloadClient   String?   @map("download_client") // qbittorrent, transmission
+  downloadClient   String?   @map("download_client") // qbittorrent, sabnzbd
   downloadClientId String?   @map("download_client_id")
   downloadStatus   String?   @map("download_status")
   // Status values: queued, downloading, completed, failed, stalled
@@ -259,6 +260,8 @@ model DownloadHistory {
 
   @@index([requestId])
   @@index([selected])
+  @@index([torrentHash])
+  @@index([nzbId])
   @@index([createdAt(sort: Desc)])
   @@map("download_history")
 }