fix: remove auto-appending of /RPC2 from RTorrentClient and finalize PDCA documentation

- Remove auto-appending of /RPC2 from RTorrentClient constructor
- Use exact URL from config (supports custom paths like whatbox.ca/xmlrpc)
- Update .env.sample with clear URL path documentation and examples
- Update README.md with comprehensive PDCA section and all download clients
- Add URL path verification tests (whatbox.ca, custom paths, no auth)
- Update architecture diagram to include Transmission and rTorrent
- Update Docker Compose example to include all download clients
- Update prerequisites to mention all supported download clients
- Update "What It Does" and "The Matching Process" sections

server/clients/RTorrentClient.js

@@ -5,18 +5,18 @@ const { logToFile } = require('../utils/logger');
 
 /**
  * rTorrent download client implementation.
- * Communicates via XML-RPC over HTTP (typically ${url}/RPC2).
+ * Communicates via XML-RPC over HTTP.
  * Supports HTTP Basic Auth when username/password are configured.
+ * The URL field must include the full XML-RPC endpoint path (e.g., http://rtorrent.local:8080/RPC2 or https://user.whatbox.ca/xmlrpc).
  */
 class RTorrentClient extends DownloadClient {
   constructor(instance) {
     super(instance);
-    this.rpcUrl = `${this.url}/RPC2`;
     this._createClient();
   }
 
   _createClient() {
-    const clientOptions = { url: this.rpcUrl };
+    const clientOptions = { url: this.url };
 
     if (this.username && this.password) {
       clientOptions.headers = {