feat: add Feishu OAuth integration

Implement Feishu OAuth provider using standard client:
- Set up Feishu-specific endpoints for authorization, token, and userinfo
- Use user_id as sub claim for Feishu user identification
- Extract correct user information from nested 'data' field in Feishu responses

Configuration requirements:
- Set FEISHU_CLIENT_ID and FEISHU_CLIENT_SECRET environment variables to enable Feishu OAuth
- Set ENABLE_OAUTH_SIGNUP=true to allow automatic user creation after OAuth login
- Set DEFAULT_USER_ROLE=user to grant immediate access after OAuth registration
- Set OAUTH_MERGE_ACCOUNTS_BY_EMAIL=true to enable merging of existing user accounts with matching emails

backend/open_webui/utils/oauth.py

@@ -602,6 +602,8 @@ class OAuthManager:
                 or (auth_manager_config.OAUTH_USERNAME_CLAIM not in user_data)
             ):
                 user_data: UserInfo = await client.userinfo(token=token)
+            if provider == "feishu" and isinstance(user_data, dict) and "data" in user_data:
+                user_data = user_data["data"]
             if not user_data:
                 log.warning(f"OAuth callback failed, user data is missing: {token}")
                 raise HTTPException(400, detail=ERROR_MESSAGES.INVALID_CRED)