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/tasks/task-pool-state.ts

@@ -0,0 +1,35 @@
+/**
+ * Task Pool State
+ *
+ * Shared state for task pool pause/resume functionality.
+ * This is kept separate to avoid circular dependencies between
+ * task-service.ts and routes/tasks.ts.
+ *
+ * State is in-memory and resets on server restart.
+ * By default, the pool is OPEN (not paused).
+ */
+
+let taskPoolPaused = false;
+
+export function isTaskPoolPaused(): boolean {
+  return taskPoolPaused;
+}
+
+export function pauseTaskPool(): void {
+  taskPoolPaused = true;
+  console.log('[TaskPool] Task pool PAUSED - workers will not pick up new tasks');
+}
+
+export function resumeTaskPool(): void {
+  taskPoolPaused = false;
+  console.log('[TaskPool] Task pool RESUMED - workers can pick up tasks');
+}
+
+export function getTaskPoolStatus(): { paused: boolean; message: string } {
+  return {
+    paused: taskPoolPaused,
+    message: taskPoolPaused
+      ? 'Task pool is paused - workers will not pick up new tasks'
+      : 'Task pool is open - workers are picking up tasks',
+  };
+}