fix: auto-fix code issues (cron)

- 修复重复导入/字段
- 修复异常处理
- 修复PEP8格式问题
- 添加类型注解

backend/rate_limiter.py

@@ -56,7 +56,7 @@ class SlidingWindowCounter:
             self._cleanup_old(now)
             return sum(self.requests.values())
 
-    def _cleanup_old(self, now: int):
+    def _cleanup_old(self, now: int) -> None:
         """清理过期的请求记录 - 使用独立锁避免竞态条件"""
         cutoff = now - self.window_size
         old_keys = [k for k in list(self.requests.keys()) if k < cutoff]
@@ -67,7 +67,7 @@ class SlidingWindowCounter:
 class RateLimiter:
     """API 限流器"""
 
-    def __init__(self):
+    def __init__(self) -> None:
         # key -> SlidingWindowCounter
         self.counters: dict[str, SlidingWindowCounter] = {}
         # key -> RateLimitConfig
@@ -143,7 +143,7 @@ class RateLimiter:
             retry_after=max(0, config.window_size) if current_count >= config.requests_per_minute else 0,
         )
 
-    def reset(self, key: str | None = None):
+    def reset(self, key: str | None = None) -> None:
         """重置限流计数器"""
         if key:
             self.counters.pop(key, None)
@@ -166,7 +166,7 @@ def get_rate_limiter() -> RateLimiter:
 
 
 # 限流装饰器（用于函数级别限流）
-def rate_limit(requests_per_minute: int = 60, key_func: Callable | None = None):
+def rate_limit(requests_per_minute: int = 60, key_func: Callable | None = None) -> None:
     """
     限流装饰器