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-service.ts

@@ -9,6 +9,7 @@
  */
 
 import { pool } from '../db/pool';
+import { isTaskPoolPaused } from './task-pool-state';
 
 // Helper to check if a table exists
 async function tableExists(tableName: string): Promise<boolean> {
@@ -149,8 +150,14 @@ class TaskService {
   /**
    * Claim a task atomically for a worker
    * If role is null, claims ANY available task (role-agnostic worker)
+   * Returns null if task pool is paused.
    */
   async claimTask(role: TaskRole | null, workerId: string): Promise<WorkerTask | null> {
+    // Check if task pool is paused - don't claim any tasks
+    if (isTaskPoolPaused()) {
+      return null;
+    }
+
     if (role) {
       // Role-specific claiming - use the SQL function
       const result = await pool.query(
@@ -170,7 +177,6 @@ class TaskService {
       WHERE id = (
         SELECT id FROM worker_tasks
         WHERE status = 'pending'
-          AND active = true
           AND (scheduled_for IS NULL OR scheduled_for <= NOW())
           -- Exclude stores that already have an active task
           AND (dispensary_id IS NULL OR dispensary_id NOT IN (