feat(tasks): Add task pool start/stop toggle

- Add task-pool-state.ts for shared pause/resume state
- Add /api/tasks/pool/status, pause, resume endpoints
- Add Start/Stop Pool toggle button to TasksDashboard
- Spinner stops when pool is closed
- Fix is_active column name in store-discovery.ts
- Fix missing active column in task-service.ts claimTask
- Auto-refresh every 15 seconds

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/routes/tasks.ts

@@ -13,6 +13,12 @@ import {
   TaskFilter,
 } from '../tasks/task-service';
 import { pool } from '../db/pool';
+import {
+  isTaskPoolPaused,
+  pauseTaskPool,
+  resumeTaskPool,
+  getTaskPoolStatus,
+} from '../tasks/task-pool-state';
 
 const router = Router();
 
@@ -592,4 +598,42 @@ router.post('/migration/full-migrate', async (req: Request, res: Response) => {
   }
 });
 
+/**
+ * GET /api/tasks/pool/status
+ * Check if task pool is paused
+ */
+router.get('/pool/status', async (_req: Request, res: Response) => {
+  const status = getTaskPoolStatus();
+  res.json({
+    success: true,
+    ...status,
+  });
+});
+
+/**
+ * POST /api/tasks/pool/pause
+ * Pause the task pool - workers won't pick up new tasks
+ */
+router.post('/pool/pause', async (_req: Request, res: Response) => {
+  pauseTaskPool();
+  res.json({
+    success: true,
+    paused: true,
+    message: 'Task pool paused - workers will not pick up new tasks',
+  });
+});
+
+/**
+ * POST /api/tasks/pool/resume
+ * Resume the task pool - workers will pick up tasks again
+ */
+router.post('/pool/resume', async (_req: Request, res: Response) => {
+  resumeTaskPool();
+  res.json({
+    success: true,
+    paused: false,
+    message: 'Task pool resumed - workers will pick up new tasks',
+  });
+});
+
 export default router;