docs: update project docs and clean up redundant files

- PROGRESS.md: update to 2026-05-29 with security hardening (T-005),
  4-frontend architecture, AI provider refactoring, discovery features,
  landing page/referral/quota, desktop layout, admin AI management
- AGENTS.md: add AI provider list (Alibaba/NVIDIA, removed Claude/DeepL/Local),
  DB-driven config, CSRF/rate-limit/CORS notes, admin_ai reload quirk
- .env.example: sync with actual config, replace deprecated providers
  with current Sensenova/OpencodeGo/NVIDIA/Spark/Alibaba
- docs/PROJECT_STATUS.md: archive (fully superseded by PROGRESS.md)
- Remove generated JS files (_bing_search.js, _batch_search.js)
- Remove empty directories (data/corpus, data/models)
- Remove backend/.coverage (test artifact)
- Fix services/.gitignore to cover _bing_search.js
- Include pending AI provider DB admin feature (admin_ai, AIProvider model,
  AIProviders.vue, migration) and T-008 test report

admin-frontend/src/router/index.js

@@ -74,6 +74,7 @@ const routes = [
     meta: { requiresAuth: true },
     children: [
       { path: '', name: 'SearchConfig', component: () => import('@/views/SearchConfig.vue'), meta: { title: '搜索配置' } },
+      { path: 'ai-providers', name: 'AIProviders', component: () => import('@/views/AIProviders.vue'), meta: { title: 'AI 模型配置' } },
     ]
   },
 ]