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

package.json

@@ -22,7 +22,8 @@
     "express": "^4.18.2",
     "express-rate-limit": "^7.0.0",
     "helmet": "^7.0.0",
-    "jsdom": "^29.1.1"
+    "jsdom": "^29.1.1",
+    "xmlrpc": "^1.3.2"
   },
   "devDependencies": {
     "@vitest/coverage-v8": "^4.1.6",