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/ARCHITECTURE.md

@@ -326,6 +326,13 @@ interface NormalizedDownload {
 - Handles session ID management and conflict resolution
 - Demonstrates how easy it is to add new client types
 
+#### RTorrentClient
+- XML-RPC implementation for rTorrent daemon
+- Uses the xmlrpc package (v1.3.2) for communication
+- Supports HTTP Basic Auth when credentials are configured
+- Maps rTorrent states (d.state, d.is_active, d.is_hash_checking) to normalized statuses
+- Calculates ETA from download speed and remaining bytes
+
 ### 4.4.5 Registry and Factory (`downloadClients.js`)
 
 The `DownloadClientRegistry` manages all client instances: