chore: post-deployment cleanup and docs update

- Make AI routing rules DB-driven (read from system_configs, removed from config.py)
- Add translation quota tracking to LLM translation (OpenAIProvider)
- Add Alibaba MT ECS RAM role support (STS token, no AccessKey needed)
- Fix admin sidebar link for AI模型配置 page
- Fix Quota.vue API path (quotas → translation-quotas)
- Fix login auto-redirect to dashboard
- Add provider dropdown selects to AI routing config UI
- Clean up stale ai_provider_* system_configs records
- Remove OpencodeGo, Spark providers (code + DB)
- Update deploy config: nginx port 8000, systemd cwd

user-frontend/src/api/index.js

@@ -116,4 +116,7 @@ export function sendWhatsApp(data) { return http.post('/whatsapp/send', data) }
 
 export function getUsageStats() { return http.get('/usage/stats') }
 
+export function aiChat(message, history = []) { return http.post('/ai/chat', { message, history }) }
+export function aiQuickQuestions() { return http.get('/ai/quick-questions') }
+
 export default http