feat: improve error handling and code quality

后端改进:
- 添加统一异常处理系统 (exceptions.py, response.py)
- 实现自定义异常类 (ValidationError, AuthorizationError, ResourceNotFoundError, BusinessLogicError)
- 配置全局异常处理器,统一 API 错误响应格式
- 迁移业务逻辑错误到自定义异常 (users.py, auth.py)
- 添加 SQL LIKE 通配符转义,防止通配符滥用
- 使用 EmailStr 进行邮箱格式验证
- 移除敏感字段暴露 (jwt_sub)

前端改进:
- 配置 ESLint 9 (flat config) 和 Prettier
- 修复所有 ESLint 错误和警告
- 移除未使用的变量和导入
- 为组件添加 PropTypes 默认值
- 统一代码格式和风格

backend/exceptions.py

@@ -0,0 +1,64 @@
+"""
+自定义异常类
+
+提供统一的异常处理机制，避免直接抛出通用Exception
+"""
+
+
+class BaseAPIException(Exception):
+    """API 异常基类"""
+
+    def __init__(self, message: str, status_code: int = 500, error_code: str = None):
+        self.message = message
+        self.status_code = status_code
+        self.error_code = error_code or self.__class__.__name__
+        super().__init__(self.message)
+
+
+class ValidationError(BaseAPIException):
+    """验证错误 - 400"""
+
+    def __init__(self, message: str, error_code: str = "VALIDATION_ERROR"):
+        super().__init__(message, status_code=400, error_code=error_code)
+
+
+class AuthenticationError(BaseAPIException):
+    """认证错误 - 401"""
+
+    def __init__(self, message: str = "未授权", error_code: str = "AUTHENTICATION_ERROR"):
+        super().__init__(message, status_code=401, error_code=error_code)
+
+
+class AuthorizationError(BaseAPIException):
+    """授权错误 - 403"""
+
+    def __init__(self, message: str = "无权限访问", error_code: str = "AUTHORIZATION_ERROR"):
+        super().__init__(message, status_code=403, error_code=error_code)
+
+
+class ResourceNotFoundError(BaseAPIException):
+    """资源未找到 - 404"""
+
+    def __init__(self, message: str = "资源未找到", error_code: str = "NOT_FOUND"):
+        super().__init__(message, status_code=404, error_code=error_code)
+
+
+class ResourceConflictError(BaseAPIException):
+    """资源冲突 - 409"""
+
+    def __init__(self, message: str = "资源冲突", error_code: str = "CONFLICT"):
+        super().__init__(message, status_code=409, error_code=error_code)
+
+
+class BusinessLogicError(BaseAPIException):
+    """业务逻辑错误 - 默认422，但可自定义状态码（如429）"""
+
+    def __init__(self, message: str, error_code: str = "BUSINESS_ERROR", status_code: int = 422):
+        super().__init__(message, status_code=status_code, error_code=error_code)
+
+
+class InternalServerError(BaseAPIException):
+    """服务器内部错误 - 500"""
+
+    def __init__(self, message: str = "服务器内部错误", error_code: str = "INTERNAL_ERROR"):
+        super().__init__(message, status_code=500, error_code=error_code)