feat: add rtorrent client via PDCA

- Implement RTorrentClient extending DownloadClient abstract class
- Use xmlrpc package (v1.3.2) for XML-RPC communication
- Support HTTP Basic Auth when credentials are configured
- Map rTorrent states (d.state, d.is_active, d.is_hash_checking) to normalized statuses
- Calculate ETA from download speed and remaining bytes
- Add getRtorrentInstances() to config.js
- Register RTorrentClient in downloadClients.js registry
- Add 8 comprehensive unit tests covering all functionality
- Update .env.sample with rtorrent configuration examples
- Update ARCHITECTURE.md with rtorrent client details
- Update ADDING-A-DOWNLOAD-CLIENT.md with rtorrent-specific notes

.env.sample

@@ -94,6 +94,20 @@ QBITTORRENT_INSTANCES=[{"name":"main","url":"https://qbittorrent.example.com","u
 # QBITTORRENT_USERNAME=admin
 # QBITTORRENT_PASSWORD=your-password
 
+# =============================================================================
+# RTORRENT INSTANCES (JSON Array Format)
+# Add one or more rTorrent instances as a single-line JSON array
+# Uses username/password authentication (optional)
+# Format: [{"name":"instance-name","url":"https://...","username":"...","password":"..."}]
+# XML-RPC endpoint is automatically appended: ${url}/RPC2
+# =============================================================================
+# RTORRENT_INSTANCES=[{"name":"main","url":"https://rtorrent.example.com","username":"rtorrent","password":"rtorrent"}]
+
+# Legacy single-instance format (optional - still supported)
+# RTORRENT_URL=https://rtorrent.example.com
+# RTORRENT_USERNAME=rtorrent
+# RTORRENT_PASSWORD=rtorrent
+
 # =============================================================================
 # SONARR INSTANCES (JSON Array Format)
 # Add one or more Sonarr instances as a single-line JSON array