prepare fastapi root layout for server deployment

app/core/exceptions.py

@@ -0,0 +1,8 @@
+class AppError(Exception):
+    """业务异常：承载业务错误码、错误信息和 HTTP 状态码。"""
+
+    def __init__(self, code: str, message: str, status_code: int = 400) -> None:
+        self.code = code
+        self.message = message
+        self.status_code = status_code
+        super().__init__(message)