Detect a dead local pipeline worker after API restart, and flush SSE.

A docker rebuild left HubAudio status=running with no process, so the UI sat on a silent event stream. Persist the worker pid and disable proxy buffering on the SSE response.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/routers/pipeline.py

@@ -471,7 +471,15 @@ async def events(project_id: str, request: Request):
             except (asyncio.CancelledError, Exception):
                 pass
 
-    return EventSourceResponse(event_generator())
+    return EventSourceResponse(
+        event_generator(),
+        ping=15,
+        headers={
+            "Cache-Control": "no-cache, no-transform",
+            "X-Accel-Buffering": "no",
+            "Connection": "keep-alive",
+        },
+    )
 
 
 @router.get("/pipeline/{project_id}/status")