feat(M13 T-519b): SSE worker_heartbeat + NavBar stand-by badge

Aanvulling op scrum4me-mcp PR #25 (worker_heartbeat MCP-tool).

- app/api/realtime/solo/route.ts: WorkerHeartbeatPayload type +
  isWorkerHeartbeatPayload guard + shouldEmit-routing op user_id.
- stores/solo-store.ts: workerQuotaPct + workerQuotaCheckAt state +
  setWorkerQuota action. Reset bij decrementWorkers naar 0.
- lib/realtime/use-solo-realtime.ts: handle worker_heartbeat-event,
  roep setWorkerQuota.
- components/solo/nav-status-indicators.tsx: stand-by badge wanneer
  workerQuotaPct < minQuotaPct + tooltip met drempel.
- components/shared/nav-bar.tsx + app/(app)/layout.tsx: minQuotaPct
  prop plumbing van User.min_quota_pct naar NavStatusIndicators.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/api/realtime/solo/route.ts

@@ -64,7 +64,17 @@ type WorkerPayload = {
   product_id?: string
 }
 
-type NotifyPayload = EntityPayload | JobPayload | WorkerPayload
+// M13: per-iteration quota-rapport van de worker. Geen product-scope —
+// elke heartbeat geldt voor alle producten waar deze user toegang toe heeft.
+type WorkerHeartbeatPayload = {
+  type: 'worker_heartbeat'
+  user_id: string
+  token_id: string
+  last_quota_pct: number
+  last_quota_check_at: string
+}
+
+type NotifyPayload = EntityPayload | JobPayload | WorkerPayload | WorkerHeartbeatPayload
 
 function isJobPayload(p: NotifyPayload): p is JobPayload {
   return 'type' in p && (p.type === 'claude_job_enqueued' || p.type === 'claude_job_status')
@@ -74,6 +84,10 @@ function isWorkerPayload(p: NotifyPayload): p is WorkerPayload {
   return 'type' in p && (p.type === 'worker_connected' || p.type === 'worker_disconnected')
 }
 
+function isWorkerHeartbeatPayload(p: NotifyPayload): p is WorkerHeartbeatPayload {
+  return 'type' in p && p.type === 'worker_heartbeat'
+}
+
 function shouldEmit(
   payload: NotifyPayload,
   productId: string,
@@ -90,6 +104,10 @@ function shouldEmit(
     return payload.user_id === userId
   }
 
+  if (isWorkerHeartbeatPayload(payload)) {
+    return payload.user_id === userId
+  }
+
   // M11 (ST-1104): question-events horen op /api/realtime/notifications, niet hier.
   if (payload.entity === 'question') return false