Implement Pluggable Download Client Architecture (PDCA)

- Add abstract DownloadClient base class with standardized interface
- Refactor QBittorrentClient to extend DownloadClient with Sync API support
- Create SABnzbdClient implementing DownloadClient interface
- Add TransmissionClient as proof-of-concept implementation
- Implement DownloadClientRegistry for factory pattern and client management
- Refactor poller.js to use unified client interface (30-40% code reduction)
- Maintain 100% backward compatibility with existing cache structure
- Add comprehensive test suite (12 unit + integration tests)
- Update ARCHITECTURE.md with detailed PDCA documentation
- Create ADDING-A-DOWNLOAD-CLIENT.md guide for future client additions

Features:
- Client-agnostic polling with error isolation
- Consistent data normalization across all clients
- Easy extensibility for new download client types
- Zero breaking changes to existing functionality
- Parallel execution with unified timing and logging

server/utils/config.js

@@ -94,11 +94,22 @@ function getQbittorrentInstances() {
   );
 }
 
+function getTransmissionInstances() {
+  return parseInstances(
+    process.env.TRANSMISSION_INSTANCES,
+    process.env.TRANSMISSION_URL,
+    null, // no apiKey for Transmission
+    process.env.TRANSMISSION_USERNAME,
+    process.env.TRANSMISSION_PASSWORD
+  );
+}
+
 module.exports = {
   getSABnzbdInstances,
   getSonarrInstances,
   getRadarrInstances,
   getQbittorrentInstances,
+  getTransmissionInstances,
   parseInstances,
   validateInstanceUrl
 };