refactor: 项目结构优化与FastAPI生命周期更新

重构项目目录结构，提高代码组织性和可维护性

将schemas目录重命名为domain，更好地表达领域模型概念
将services目录细分为service/chat、service/image等子目录
将api目录重命名为router，更符合FastAPI惯例
创建utils目录存放通用工具函数
更新FastAPI应用程序生命周期管理

替换已弃用的on_event方法为推荐的lifespan事件处理器
添加应用程序关闭时的日志记录
代码质量改进

抽取常量到constants.py，减少硬编码值
添加helpers.py提供通用工具函数
优化配置管理，使用环境变量和默认值
完善文档字符串，提高代码可读性

app/exception/exceptions.py

@@ -0,0 +1,133 @@
+"""
+异常处理模块，定义应用程序中使用的自定义异常和异常处理器
+"""
+from fastapi import Request, FastAPI
+from fastapi.responses import JSONResponse
+from fastapi.exceptions import RequestValidationError
+from starlette.exceptions import HTTPException as StarletteHTTPException
+
+from app.logger.logger import get_main_logger
+
+logger = get_main_logger()
+
+
+class APIError(Exception):
+    """API错误基类"""
+    def __init__(self, status_code: int, detail: str, error_code: str = None):
+        self.status_code = status_code
+        self.detail = detail
+        self.error_code = error_code or "api_error"
+        super().__init__(self.detail)
+
+
+class AuthenticationError(APIError):
+    """认证错误"""
+    def __init__(self, detail: str = "Authentication failed"):
+        super().__init__(status_code=401, detail=detail, error_code="authentication_error")
+
+
+class AuthorizationError(APIError):
+    """授权错误"""
+    def __init__(self, detail: str = "Not authorized to access this resource"):
+        super().__init__(status_code=403, detail=detail, error_code="authorization_error")
+
+
+class ResourceNotFoundError(APIError):
+    """资源未找到错误"""
+    def __init__(self, detail: str = "Resource not found"):
+        super().__init__(status_code=404, detail=detail, error_code="resource_not_found")
+
+
+class ModelNotSupportedError(APIError):
+    """模型不支持错误"""
+    def __init__(self, model: str):
+        super().__init__(
+            status_code=400, 
+            detail=f"Model {model} is not supported", 
+            error_code="model_not_supported"
+        )
+
+
+class APIKeyError(APIError):
+    """API密钥错误"""
+    def __init__(self, detail: str = "Invalid or expired API key"):
+        super().__init__(status_code=401, detail=detail, error_code="api_key_error")
+
+
+class ServiceUnavailableError(APIError):
+    """服务不可用错误"""
+    def __init__(self, detail: str = "Service temporarily unavailable"):
+        super().__init__(status_code=503, detail=detail, error_code="service_unavailable")
+
+
+def setup_exception_handlers(app: FastAPI) -> None:
+    """
+    设置应用程序的异常处理器
+    
+    Args:
+        app: FastAPI应用程序实例
+    """
+    @app.exception_handler(APIError)
+    async def api_error_handler(request: Request, exc: APIError):
+        """处理API错误"""
+        logger.error(f"API Error: {exc.detail} (Code: {exc.error_code})")
+        return JSONResponse(
+            status_code=exc.status_code,
+            content={
+                "error": {
+                    "code": exc.error_code,
+                    "message": exc.detail
+                }
+            }
+        )
+    
+    @app.exception_handler(StarletteHTTPException)
+    async def http_exception_handler(request: Request, exc: StarletteHTTPException):
+        """处理HTTP异常"""
+        logger.error(f"HTTP Exception: {exc.detail} (Status: {exc.status_code})")
+        return JSONResponse(
+            status_code=exc.status_code,
+            content={
+                "error": {
+                    "code": "http_error",
+                    "message": exc.detail
+                }
+            }
+        )
+    
+    @app.exception_handler(RequestValidationError)
+    async def validation_exception_handler(request: Request, exc: RequestValidationError):
+        """处理请求验证错误"""
+        error_details = []
+        for error in exc.errors():
+            error_details.append({
+                "loc": error["loc"],
+                "msg": error["msg"],
+                "type": error["type"]
+            })
+        
+        logger.error(f"Validation Error: {error_details}")
+        return JSONResponse(
+            status_code=422,
+            content={
+                "error": {
+                    "code": "validation_error",
+                    "message": "Request validation failed",
+                    "details": error_details
+                }
+            }
+        )
+    
+    @app.exception_handler(Exception)
+    async def general_exception_handler(request: Request, exc: Exception):
+        """处理通用异常"""
+        logger.exception(f"Unhandled Exception: {str(exc)}")
+        return JSONResponse(
+            status_code=500,
+            content={
+                "error": {
+                    "code": "internal_server_error",
+                    "message": "An unexpected error occurred"
+                }
+            }
+        )