Update RetCode to common.constants (#10984)

### What problem does this PR solve?

1. Update RetCode to common.constants
2. Decouple the admin and API modules

### Type of change

- [x] Refactoring

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

api/settings.py

@@ -17,7 +17,6 @@ import json
 import os
 import secrets
 from datetime import date
-from enum import Enum, IntEnum
 
 import rag.utils
 import rag.utils.es_conn
@@ -205,41 +204,6 @@ def init_settings():
     MAIL_FRONTEND_URL = SMTP_CONF.get("mail_frontend_url", "")
 
 
-class CustomEnum(Enum):
-    @classmethod
-    def valid(cls, value):
-        try:
-            cls(value)
-            return True
-        except BaseException:
-            return False
-
-    @classmethod
-    def values(cls):
-        return [member.value for member in cls.__members__.values()]
-
-    @classmethod
-    def names(cls):
-        return [member.name for member in cls.__members__.values()]
-
-
-class RetCode(IntEnum, CustomEnum):
-    SUCCESS = 0
-    NOT_EFFECTIVE = 10
-    EXCEPTION_ERROR = 100
-    ARGUMENT_ERROR = 101
-    DATA_ERROR = 102
-    OPERATING_ERROR = 103
-    CONNECTION_ERROR = 105
-    RUNNING = 106
-    PERMISSION_ERROR = 108
-    AUTHENTICATION_ERROR = 109
-    UNAUTHORIZED = 401
-    SERVER_ERROR = 500
-    FORBIDDEN = 403
-    NOT_FOUND = 404
-
-
 def _parse_model_entry(entry):
     if isinstance(entry, str):
         return {"name": entry, "factory": None, "api_key": None, "base_url": None}