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/layouts/UserLayout.vue

@@ -102,6 +102,7 @@
         </div>
       </footer>
     </div>
+    <AiAssistant />
   </div>
 </template>
 
@@ -110,6 +111,7 @@ import { ref, computed, onMounted } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import { useAuthStore } from '@/stores/auth'
 import { getUnreadCount } from '@/api'
+import AiAssistant from '@/components/AiAssistant.vue'
 
 const route = useRoute()
 const router = useRouter()