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

uni-app/src/utils/api.js

@@ -176,6 +176,11 @@ export const adminApi = {
   updateConfig: (key, value) => request(`/admin/config/${key}`, 'PUT', { value }),
 }
 
+export const aiChatApi = {
+  chat: (message, history = []) => request('/ai/chat', 'POST', { message, history }),
+  quickQuestions: () => request('/ai/quick-questions'),
+}
+
 export const analyticsApi = {
   getOverview: () => request('/analytics/overview'),
   getCustomers: () => request('/analytics/customers'),