fix: /login/admin 自动把历史 admin 账户升级为 admin 角色

历史上以 username=admin 自动注册的账户曾被建成 system_role=user，
登录时按 username 修正一次，避免老库需要手工改表。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

offline-backend-20260512/backend/app/gateway/routers/auth.py

@@ -379,6 +379,13 @@ async def login_by_username(request: Request, response: Response, body: Username
             user = await provider.get_user_by_email(email)
             if user is None:
                 raise HTTPException(status_code=status.HTTP_500_INTERNAL_SERVER_ERROR, detail="Failed to create or load user")
+    elif body.username == "admin" and user.system_role != "admin":
+        # Historical accounts created before the username-based role assignment
+        # may still have system_role="user" — promote the canonical admin user
+        # so /login/admin behaves consistently.
+        user.system_role = "admin"
+        user = await provider.update_user(user)
+        logger.info("Promoted historical 'admin' account to system_role=admin: %s", email)
 
     token = create_access_token(str(user.id), token_version=user.token_version)
     _set_session_cookie(response, token, request)