Fix stuck 'processing' tasks after restart

On startup, tasks in downloading/processing state are interrupted (not
completed). Previously they were re-queued, causing the same task to be
processed multiple times after each restart.

Now: interrupted tasks are immediately marked as error with message
'服务重启，任务中断'. Only 'pending' tasks are re-queued.

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

web_app.py

@@ -331,9 +331,14 @@ def _load_tasks_from_db():
                 "completed_at": r.get("completed_at"),
                 "logs": [],
             }
+            # 重启时 downloading/processing 说明上次处理被中断，标为 error
+            if task["status"] in ("downloading", "processing"):
+                task["status"] = "error"
+                task["error"] = "服务重启，任务中断"
+                _save_task_to_db(task)
             with task_lock:
                 tasks[task["id"]] = task
-            if task["status"] in ("pending", "downloading", "processing"):
+            if task["status"] == "pending":
                 task_queue.put(task["id"])
         conn.close()
     except Exception as e: