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

tests/unit/clients/RTorrentClient.test.js

@@ -39,12 +39,11 @@ describe('RTorrentClient', () => {
       expect(client.getInstanceId()).toBe('test-rtorrent');
       expect(client.name).toBe('Test rTorrent');
       expect(client.url).toBe('http://localhost:8080');
-      expect(client.rpcUrl).toBe('http://localhost:8080/RPC2');
     });
 
-    it('should create xmlrpc client with basic auth when credentials provided', () => {
+    it('should create xmlrpc client with exact URL from config (no auto-append)', () => {
       expect(xmlrpc.createClient).toHaveBeenCalledWith({
-        url: 'http://localhost:8080/RPC2',
+        url: 'http://localhost:8080',
         headers: {
           Authorization: `Basic ${Buffer.from('rtorrent:rtorrent').toString('base64')}`
         }
@@ -56,13 +55,44 @@ describe('RTorrentClient', () => {
       const noAuthConfig = {
         id: 'test-rtorrent-noauth',
         name: 'Test rTorrent No Auth',
-        url: 'http://localhost:8080'
+        url: 'http://localhost:8080/RPC2'
       };
       new RTorrentClient(noAuthConfig);
       expect(xmlrpc.createClient).toHaveBeenCalledWith({
         url: 'http://localhost:8080/RPC2'
       });
     });
+
+    it('should use whatbox.ca-style /xmlrpc path exactly as configured', () => {
+      xmlrpc.createClient.mockClear();
+      const whatboxConfig = {
+        id: 'test-whatbox',
+        name: 'Whatbox',
+        url: 'https://user.whatbox.ca/xmlrpc',
+        username: 'user',
+        password: 'pass'
+      };
+      new RTorrentClient(whatboxConfig);
+      expect(xmlrpc.createClient).toHaveBeenCalledWith({
+        url: 'https://user.whatbox.ca/xmlrpc',
+        headers: {
+          Authorization: `Basic ${Buffer.from('user:pass').toString('base64')}`
+        }
+      });
+    });
+
+    it('should use custom RPC path exactly as configured', () => {
+      xmlrpc.createClient.mockClear();
+      const customConfig = {
+        id: 'test-custom',
+        name: 'Custom',
+        url: 'https://example.com/custom/rpc/path'
+      };
+      new RTorrentClient(customConfig);
+      expect(xmlrpc.createClient).toHaveBeenCalledWith({
+        url: 'https://example.com/custom/rpc/path'
+      });
+    });
   });
 
   describe('Connection Test', () => {