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

docs/ADDING-A-DOWNLOAD-CLIENT.md

@@ -346,6 +346,18 @@ For a complete example, refer to the existing client implementations:
 - **SABnzbdClient.js**: Simple REST API client
 - **QBittorrentClient.js**: Complex client with sync API and fallback
 - **TransmissionClient.js**: JSON-RPC client with session management
+- **RTorrentClient.js**: XML-RPC client with HTTP Basic Auth
+
+### rTorrent Specific Notes
+
+rTorrent uses XML-RPC over HTTP with the following specifics:
+
+- **Endpoint**: `${url}/RPC2` (most common)
+- **Authentication**: HTTP Basic Auth (handled by reverse proxy or web server)
+- **Primary Method**: `d.multicall2` for efficient bulk torrent data retrieval
+- **Library**: Uses the `xmlrpc` package (v1.3.2)
+- **Status Mapping**: Combines `d.state`, `d.is_active`, and `d.is_hash_checking` to determine status
+- **ETA Calculation**: Computed from download speed and remaining bytes when actively downloading
 
 ## Troubleshooting