Fix SSE keepalive: yield explicit ping event, enforce test timeout

- event_generator now yields {event: 'ping', data: ''} on KEEPALIVE_TIMEOUT
  instead of silently looping. Gives SSE clients stream-level liveness signal.
- _collect_events helper now enforces its timeout parameter via asyncio.wait_for,
  preventing tests from hanging indefinitely if generator never yields.

backend/app/routers/sse.py

@@ -70,8 +70,9 @@ async def event_generator(
                         "data": json.dumps(event.model_dump()),
                     }
             except asyncio.TimeoutError:
-                # No event in KEEPALIVE_TIMEOUT — loop back and wait again.
-                # sse-starlette's built-in ping handles the actual keepalive.
+                # Yield an explicit ping so SSE clients see stream liveness
+                # (in addition to sse-starlette's built-in TCP keepalive).
+                yield {"event": "ping", "data": ""}
                 continue
     finally:
         broker.unsubscribe(session_id, queue)

backend/tests/test_sse.py

@@ -37,10 +37,14 @@ def _make_job(session_id: str, *, status: str = "queued", **overrides) -> Job:
 async def _collect_events(gen, *, count: int = 1, timeout: float = 5.0):
     """Consume *count* events from an async generator with a safety timeout."""
     events = []
+
+    async def _drain():
         async for event in gen:
             events.append(event)
             if len(events) >= count:
                 break
+
+    await asyncio.wait_for(_drain(), timeout=timeout)
     return events