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

server/utils/config.js

@@ -104,12 +104,23 @@ function getTransmissionInstances() {
   );
 }
 
+function getRtorrentInstances() {
+  return parseInstances(
+    process.env.RTORRENT_INSTANCES,
+    process.env.RTORRENT_URL,
+    null, // no apiKey for rtorrent
+    process.env.RTORRENT_USERNAME,
+    process.env.RTORRENT_PASSWORD
+  );
+}
+
 module.exports = {
   getSABnzbdInstances,
   getSonarrInstances,
   getRadarrInstances,
   getQbittorrentInstances,
   getTransmissionInstances,
+  getRtorrentInstances,
   parseInstances,
   validateInstanceUrl
 };