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.

ui/src/components/features/DataFlowView.tsx

@@ -206,10 +206,43 @@ export function DataFlowView() {
   }, [genRows, load])
 
   const flowMode = (graph as unknown as { flow?: string })?.flow ?? 'running'
+
+  const heartbeat = useCallback((active: boolean) => {
+    return fetch('/api/federated/live/heartbeat', {
+      method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ active }),
+    }).catch(() => {})
+  }, [])
+
   const onFlow = useCallback(async (action: 'pause' | 'resume' | 'stop') => {
     await setStreamingFlow(action)
-    setTimeout(() => load(true), 300)
-  }, [load])
+    // The master pulse also drives the live data generator: Run starts the
+    // stream into the sources, Pause/Stop halts it.
+    await heartbeat(action === 'resume')
+    if (action === 'resume') {
+      try {
+        const r = await fetch('/api/federated/generate', {
+          method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ rows: 800 }),
+        })
+        const d = await r.json()
+        const i = d?.inserted || {}
+        setGenToast(`Generator running — seeded +${(i.orders ?? 0).toLocaleString()} orders, +${(i.telemetry ?? 0).toLocaleString()} telemetry, +${(i.hr_events ?? 0).toLocaleString()} HR, +${(i.supply_events ?? 0).toLocaleString()} supply; now streaming every few seconds via CDC`)
+        setTimeout(() => setGenToast(null), 7000)
+      } catch { /* ignore */ }
+    }
+    setTimeout(() => load(true), 400)
+  }, [load, heartbeat])
+
+  // While the flow is running and this tab is open, keep the generator alive
+  // (it stops a few seconds after you pause/stop or leave the tab).
+  useEffect(() => {
+    if (flowMode !== 'running') {
+      heartbeat(false)
+      return
+    }
+    heartbeat(true)
+    const t = setInterval(() => heartbeat(true), 5000)
+    return () => clearInterval(t)
+  }, [flowMode, heartbeat])
 
   const [maskBusy, setMaskBusy] = useState<string | null>(null)
   const onToggleMask = useCallback(async (key: string, column: string, masked: boolean) => {
@@ -334,7 +367,7 @@ export function DataFlowView() {
             >
               <FileCode2 className="h-3 w-3" /> Scripts
             </button>
-            <div className="inline-flex items-center gap-0.5 rounded border border-border p-0.5" title="Master pulse control">
+            <div className="inline-flex items-center gap-0.5 rounded border border-border p-0.5" title="Master pulse + live generator: Run streams data into the sources, Pause/Stop halts it">
               <button type="button" onClick={() => onFlow('resume')}
                 className={cn('inline-flex items-center gap-1 rounded px-1.5 py-0.5 text-[9px] font-medium transition-colors', flowMode === 'running' ? 'bg-emerald-500/25 text-emerald-200' : 'text-foreground-muted hover:text-foreground')}>
                 <Play className="h-3 w-3" /> Run