feat(api): serve long-lived streams on the event loop

Move GET /api/events, the remote-device command stream and artifact
downloads from Flask to Starlette routes mounted ahead of the Flask
catch-all. On Flask each held an a2wsgi threadpool slot for as long as
its response stayed open, and because uvicorn drops writes after a
client disconnects, a closed tab never released it.

- asgi_auth: one JWT/OIDC gate for Starlette routes; the chat reconnect
  reader uses it too
- ClosingStreamingResponse closes the body iterator and releases the
  SSE slot or file handle even when the client leaves before the first
  frame
- AsyncTopic liveness probe replaces the sync client's socket_timeout
  guard against half-open pub/sub sockets
- ASYNC_REDIS_MAX_CONNECTIONS sizes the async Redis pool; every open
  stream holds a connection and redis-py defaults to 100

docs/content/Deploying/Development-Environment.mdx

@@ -133,7 +133,7 @@ To run the DocsGPT backend locally, you'll need to set up a Python environment a
     flask --app docsgpt/app.py run --host=0.0.0.0 --port=7091
     ```
 
-    But it serves **only** the WSGI Flask app and omits the routes mounted on the ASGI shell in `docsgpt/asgi.py`: the `/mcp` FastMCP endpoint and the native-async SSE reconnect reader `GET /api/messages/<id>/events`. Under `flask run` those paths return 404 — chat still works (`POST /stream` is a Flask route), but a stream interrupted by a disconnect won't auto-resume on reconnect. Use `flask run` only when you don't need those routes.
+    But it serves **only** the WSGI Flask app and omits the native-async routes mounted on the ASGI shell in `docsgpt/asgi.py`: the `/mcp` FastMCP endpoint, the chat reconnect reader `GET /api/messages/<id>/events`, the notification stream `GET /api/events`, the remote-device command stream `GET /api/devices/sessions/<id>/events`, and artifact downloads `GET /api/artifacts/<id>/download`. Under `flask run` those paths return 404 — chat still works (`POST /stream` is a Flask route), but live notifications, stream auto-resume, paired devices and artifact downloads don't. Use `flask run` only when you don't need them.
 
 6.  **Start the Celery Worker:**