feat: Complete frontend integration with native WebSocket and FastAPI backend

- Created websocket_client.js: Native WebSocket wrapper with Socket.IO-compatible interface
  - Automatic reconnection with exponential backoff
  - Room-based subscriptions for targeted updates
  - Message queueing during disconnection

- Updated HTML templates (index.html, queue.html):
  - Replaced Socket.IO CDN with native websocket_client.js
  - No external dependencies needed

- Updated JavaScript files (app.js, queue.js):
  - Added room subscriptions on WebSocket connect (scan_progress, download_progress, downloads)
  - Added dual event handlers for backward compatibility
  - Support both old (scan_completed) and new (scan_complete) message types
  - Support both old (download_error) and new (download_failed) message types
  - Support both old (queue_updated) and new (queue_status) message types

- Registered anime router in fastapi_app.py:
  - Added anime_router import and registration
  - All API routers now properly included

- Documentation:
  - Created FRONTEND_INTEGRATION.md with comprehensive integration guide
  - Updated infrastructure.md with frontend integration section
  - Updated instructions.md to mark task as completed

- Testing:
  - Verified anime endpoint tests pass (pytest)
  - API endpoint mapping documented
  - WebSocket message format changes documented

Benefits:
  - Native WebSocket API (faster, smaller footprint)
  - No external CDN dependencies
  - Full backward compatibility with existing code
  - Proper integration with backend services
  - Real-time updates via room-based messaging

src/server/web/templates/queue.html

@@ -245,7 +245,7 @@
     </div>
 
     <!-- Scripts -->
-    <script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/4.0.1/socket.io.js"></script>
+    <script src="/static/js/websocket_client.js"></script>
     <script src="/static/js/queue.js"></script>
 </body>