feat: Parallelized store discovery, modification tracking, and task deduplication

Store Discovery Parallelization:
- Add store_discovery_state handler for per-state parallel discovery
- Add POST /api/tasks/batch/store-discovery endpoint
- 8 workers can now process states in parallel (~30-45 min vs 3+ hours)

Modification Tracking (Migration 090):
- Add last_modified_at, last_modified_by_task, last_modified_task_id to dispensaries
- Add same columns to store_products
- Update all handlers to set tracking info on modifications

Stale Task Recovery:
- Add periodic stale cleanup every 10 minutes (worker-0 only)
- Prevents orphaned tasks from blocking queue after worker crashes

Task Deduplication:
- createStaggeredTasks now skips if pending/active task exists for same role
- Skips if same role completed within last 4 hours
- API responses include skipped count

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

backend/src/tasks/handlers/product-discovery-http.ts

@@ -326,13 +326,16 @@ export async function handleProductDiscoveryHttp(ctx: TaskContext): Promise<Task
     console.log(`[ProductDiscoveryHTTP] Saved payload #${payloadResult.id} (${(payloadResult.sizeBytes / 1024).toFixed(1)}KB)`);
 
     // ============================================================
-    // STEP 6: Update dispensary last_fetch_at
+    // STEP 6: Update dispensary last_fetch_at and tracking
     // ============================================================
     await pool.query(`
       UPDATE dispensaries
-      SET last_fetch_at = NOW()
+      SET last_fetch_at = NOW(),
+          last_modified_at = NOW(),
+          last_modified_by_task = $2,
+          last_modified_task_id = $3
       WHERE id = $1
-    `, [dispensaryId]);
+    `, [dispensaryId, task.role, task.id]);
 
     // ============================================================
     // STEP 7: Queue product_refresh task to process the payload