improved SSE queue handling on shutdown

router.py

@@ -269,6 +269,11 @@ async def publish_snapshot():
                 continue
             await q.put(snapshot)
 
+async def close_all_sse_queues():
+    for q in list(_subscribers):
+        # sentinel value that the generator will recognise
+        await q.put(None)
+
 # ------------------------------------------------------------------
 # Subscriber helpers
 # ------------------------------------------------------------------
@@ -1331,6 +1336,8 @@ async def usage_stream(request: Request):
                 if await request.is_disconnected():
                     break
                 data = await queue.get()
+                if data is None:
+                    break
                 # Send the data as a single SSE message
                 yield f"data: {data}\n\n"
         finally:
@@ -1361,4 +1368,5 @@ async def startup_event() -> None:
 
 @app.on_event("shutdown")
 async def shutdown_event() -> None:
+    await close_all_sse_queues()
     await app_state["session"].close()