fix(janitor): 禁止启动时因目录不存在而自动删除数据库任务记录

reconcile_tasks_on_startup() 中有一段逻辑：遍历所有任务，如果 output_dir
在磁盘上不存在，直接 DELETE FROM tasks。这导致重启时如果目录被移动/重命名/
被之前的 janitor 清理，所有任务记录静默丢失。

修改为仅输出 WARNING 日志，保留数据库记录。用户可通过 DELETE /task/<id> 手动清理。

src/gasflux/janitor.py

@@ -310,8 +310,7 @@ def reconcile_tasks_on_startup():
                 except Exception as e:
                     current_app.logger.error(f"Failed to delete failed task {task_id}: {str(e)}", exc_info=True)
 
-        # Check for tasks with output directories that no longer exist
-        # This helps clean up database entries for manually deleted directories
+        # 检查 output_dir 不存在的任务（仅警告，不自动删除）
         rows = conn.execute("""
             SELECT task_id, output_dir
             FROM tasks
@@ -321,18 +320,21 @@ def reconcile_tasks_on_startup():
 
         orphaned_count = 0
         for row in rows:
-            task_id_from_db = row[0]  # task_id是第一个字段
-            output_dir_from_db = row[1]  # output_dir是第二个字段
+            task_id_from_db = row[0]
+            output_dir_from_db = row[1]
 
             if not Path(output_dir_from_db).exists():
-                conn.execute(
-                    "DELETE FROM tasks WHERE task_id = ?",
-                    (task_id_from_db,)
+                current_app.logger.warning(
+                    f"Startup reconciliation: Task {task_id_from_db} output directory "
+                    f"not found on disk: {output_dir_from_db}. Database record preserved."
                 )
                 orphaned_count += 1
 
         if orphaned_count > 0:
-            current_app.logger.info(f"Startup reconciliation: Marked {orphaned_count} tasks as deleted (directories not found)")
+            current_app.logger.info(
+                f"Startup reconciliation: Found {orphaned_count} tasks with missing directories "
+                f"(records preserved, use DELETE /task/<id> to manually clean up)"
+            )
 
         # Backfill output_dir for rows with NULL, using OUTPUT_FOLDER/<task_id>
         output_base = Path(current_app.config.get('OUTPUT_FOLDER') or '')