fix(monitoring): break infinite loop on invalid UPID parsing

When Proxmox API returns 400 "unable to parse worker upid", the monitoring
worker was getting empty status {}, treating it as running, and retrying
forever every 5 seconds, causing 100% CPU and log spam.

Now when get_task_status() returns empty dict, we immediately mark the task
as failed with exit_status="Invalid UPID" and move on instead of looping.

Fixes production deployment hang.

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

backend/app/workers/monitoring_worker.py

@@ -945,7 +945,20 @@ class MonitoringWorker:
 
                     # --- status -----------------------------------------------
                     proxmox_status = client.get_task_status(task.node, task.upid)
-                    pve_status = proxmox_status.get("status", "running") if proxmox_status else "running"
+                    # If status is empty, it means Proxmox API rejected the UPID
+                    if not proxmox_status:
+                        logger.warning(f"[UPID SYNC] Cannot fetch status for task #{task.id} (UPID={task.upid[:30]}...) — marking as failed")
+                        task.status = "failed"
+                        task.exit_status = "Invalid UPID"
+                        task.completed_at = utcnow()
+                        db.commit()
+                        try:
+                            broadcast_task_update(task.user_id, "task_update", task.to_dict())
+                        except Exception:
+                            pass
+                        continue
+
+                    pve_status = proxmox_status.get("status", "running")
 
                     # --- log --------------------------------------------------
                     try: