fix: _handle_pull_request 补充 synchronize action dispatch

姜维排查发现 _handle_pull_request 只处理 opened/closed，
Gitea 发 pull_request + action=synchronize 时被静默丢弃。
_handle_pr_synchronize 已存在但未被 dispatch 到。

修复：加 elif action == synchronize dispatch。
pull_request_sync 注册保留作为双保险。

src/api/toolchain_routes.py

@@ -342,6 +342,8 @@ async def _handle_pull_request(payload: Dict[str, Any]) -> None:
         await _handle_pr_opened(payload)
     elif action == "closed":
         await _handle_pr_closed(payload)
+    elif action == "synchronize":
+        await _handle_pr_synchronize(payload)
 
 
 async def _handle_pr_opened(payload: Dict[str, Any]) -> None: