fix(ci): Remove buildx cache and add preflight enforcement

- Remove cache_from/cache_to from CI (plugin bug splitting commas)
- Add preflight() method to CrawlRotator - tests proxy + anti-detect
- Add pre-task preflight check - workers MUST pass before executing
- Add releaseTask() to release tasks back to pending on preflight fail
- Rename proxy_test task to whoami for clarity

🤖 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

@@ -32,7 +32,7 @@ export type TaskRole =
   | 'payload_fetch'      // NEW: Fetches from API, saves to disk
   | 'product_refresh'    // CHANGED: Now reads from local payload
   | 'analytics_refresh'
-  | 'proxy_test';        // Tests proxy connectivity via ipify
+  | 'whoami';            // Tests proxy + anti-detect connectivity
 
 export type TaskStatus =
   | 'pending'
@@ -231,6 +231,24 @@ class TaskService {
     );
   }
 
+  /**
+   * Release a claimed task back to pending (e.g., when preflight fails)
+   * This allows another worker to pick it up.
+   */
+  async releaseTask(taskId: number): Promise<void> {
+    await pool.query(
+      `UPDATE worker_tasks
+       SET status = 'pending',
+           worker_id = NULL,
+           claimed_at = NULL,
+           started_at = NULL,
+           updated_at = NOW()
+       WHERE id = $1 AND status IN ('claimed', 'running')`,
+      [taskId]
+    );
+    console.log(`[TaskService] Task ${taskId} released back to pending`);
+  }
+
   /**
    * Mark a task as failed, with auto-retry if under max_retries
    * Returns true if task was re-queued for retry, false if permanently failed