feat: AI assistant phase 2 - configurable prompt, action operations, FAQ matching, NVIDIA provider

- Admin-configurable AI prompt/quick questions from system_configs DB
- GET /api/v1/ai/quick-questions endpoint for fetching quick questions
- Local FAQ matching for instant responses (avoid AI calls for common Qs)
- AI action extraction: "add customer" intent detected, structured data returned
- Frontend action confirmation card with editable fields, calls customer API on confirm
- NVIDIA provider (stepfun-ai/step-3.5-flash) for faster chat vs deepseek-v4-flash
- Fixed httpx client timeout preventing backend hangs
- Added log_usage calls for auth events (register/login/guest/wechat)
- Admin tabs (users/stats/logs/config) fully functional with real backend
- AiAssistant component added to all tabbar pages

backend/app/ai/base.py

@@ -31,6 +31,9 @@ class AIProvider(ABC):
     ) -> Dict[str, Any]:
         pass
 
+    async def chat(self, message: str, history: list = None, system_prompt: str = None) -> Dict[str, Any]:
+        raise NotImplementedError
+
     @property
     @abstractmethod
     def name(self) -> str: