fix: add SSE heartbeat to prevent Railway proxy timeout

Railway kills idle connections after ~30s. During long LLM calls
between agent stages, the SSE stream goes silent and gets dropped.
Now sends heartbeat events every 15s to keep the connection alive.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -285,7 +285,11 @@ async def stream_analysis(analysis_id: str):
 
     async def event_generator():
         while True:
-            event = await q.get()
+            try:
+                event = await asyncio.wait_for(q.get(), timeout=15)
+            except asyncio.TimeoutError:
+                yield {"event": "heartbeat", "data": ""}
+                continue
             if event is None:
                 break
             yield {"data": json.dumps(event)}