fix: Run control on Data Flow now starts the live data generator

Previously Run/Pause/Stop only drove the pulse animation, so on the Data Flow
tab nothing was generated (the streamer is heartbeat-gated and only the Live
dashboard was sending heartbeats). Now:
- New POST /api/federated/live/heartbeat keep-alive endpoint.
- Run seeds an immediate burst into the sources and keeps the generator alive
  (5s heartbeat) while the tab is open; Pause/Stop halts generation.

api/trino_federated.py

@@ -622,6 +622,19 @@ async def toggle_generator(body: dict = Body(default={})):
     return {"ok": True, "enabled": _GEN["enabled"], "interval": _GEN["interval"], "running": _GEN["running"]}
 
 
+@router.post("/live/heartbeat")
+async def live_heartbeat(body: dict = Body(default={})):
+    """Keep-alive for the continuous generator from any watching view (e.g. the
+    Data Flow tab's Run control). active=true → generate; active=false → stop."""
+    active = bool(body.get("active", True))
+    if active:
+        _GEN["enabled"] = True
+        _GEN["last_seen"] = time.time()
+    else:
+        _GEN["last_seen"] = 0.0
+    return {"ok": True, "running": _GEN["running"], "enabled": _GEN["enabled"]}
+
+
 @router.post("/generate")
 async def generate_now(body: dict = Body(default={})):
     """Manual one-shot burst into the source systems (the Data Flow "Generate