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/services/admin.py

@@ -298,6 +298,8 @@ class AdminService:
             SystemConfig(key="feature_registration", value={"enabled": True}, description="新用户注册开关"),
             SystemConfig(key="free_daily_limits", value={"translate_chars": 5000, "replies": 20, "marketing": 5, "customers": 5, "products": 1, "quotations": 3}, description="免费版每日配额"),
             SystemConfig(key="pro_daily_limits", value={"translate_chars": 50000, "replies": 200, "marketing": 50, "customers": 100, "products": 20, "quotations": 30}, description="Pro 版每日配额"),
+            SystemConfig(key="ai_assistant_prompt", value="你是 TradeMate（外贸小助手）的 AI 助手。你的职责是帮助外贸从业者解答关于本工具使用的问题，以及提供外贸业务建议。\n你可以回答的问题包括：\n- 功能介绍：翻译、客户管理、产品管理、报价单、营销文案、WhatsApp 集成等\n- 使用帮助：如何添加客户、如何生成报价单、如何导出数据等\n- 外贸知识：贸易术语（FOB、CIF 等）、谈判技巧、跟进策略等\n\n回答要求：\n- 简洁扼要，用中文回答\n- 涉及操作步骤时用数字列表说明\n- 不确定的问题不要编造，直接说需要查证\n- 语气友好专业", description="AI 助手系统提示词"),
+            SystemConfig(key="ai_assistant_quick_questions", value=["TradeMate 有哪些功能？", "如何添加客户？", "如何生成报价单？", "怎么导出客户数据？", "营销文案怎么生成？", "什么是 FOB、CIF？"], description="AI 助手快捷提问列表"),
         ]
         for cfg in defaults:
             self.db.add(cfg)