When a model is selected from Dify, set provider='dify' and call
through Dify Chat API (/v1/chat-messages) using the existing
DIFY_CHAT_APP_API_KEY. No separate API key configuration needed.
- Frontend: hide API Key/Base URL fields when provider is 'dify',
show 'via Dify, no API key needed' tag
- Backend: add _call_dify() to route through Dify Chat API
- Backend: _try_load_db_config supports dify provider without api_key
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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