feat: dSYM 按上传配置一并保存到远端，本地清理后回退到远端地址下载

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/database.py

@@ -25,6 +25,7 @@ def _migrate_db():
         ("tasks", "branch", "VARCHAR DEFAULT 'main'"),
         ("tasks", "error_category", "VARCHAR"),
         ("tasks", "superseded_at", "TIMESTAMP"),
+        ("tasks", "dsym_url", "VARCHAR"),
     ]
     with engine.connect() as conn:
         for table, column, col_type in migrations: