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

.env.sample

@@ -99,12 +99,18 @@ QBITTORRENT_INSTANCES=[{"name":"main","url":"https://qbittorrent.example.com","u
 # Add one or more rTorrent instances as a single-line JSON array
 # Uses username/password authentication (optional)
 # Format: [{"name":"instance-name","url":"https://...","username":"...","password":"..."}]
-# XML-RPC endpoint is automatically appended: ${url}/RPC2
+# IMPORTANT: XML-RPC endpoint must be included in the url field (no automatic appending).
+# Standard installs use /RPC2. Some providers (e.g. whatbox.ca) use /xmlrpc. Other
+# installations may use a custom path. Always supply the complete RPC endpoint.
+# Examples:
+#   Standard: http://rtorrent.local:8080/RPC2
+#   whatbox.ca: https://user.whatbox.ca/xmlrpc
+#   Custom: https://example.com/custom/rpc/path
 # =============================================================================
-# RTORRENT_INSTANCES=[{"name":"main","url":"https://rtorrent.example.com","username":"rtorrent","password":"rtorrent"}]
+# RTORRENT_INSTANCES=[{"name":"main","url":"http://rtorrent.example.com/RPC2","username":"rtorrent","password":"rtorrent"}]
 
 # Legacy single-instance format (optional - still supported)
-# RTORRENT_URL=https://rtorrent.example.com
+# RTORRENT_URL=http://rtorrent.example.com/RPC2
 # RTORRENT_USERNAME=rtorrent
 # RTORRENT_PASSWORD=rtorrent