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- docs/PROJECT_STATUS.md: archive (fully superseded by PROGRESS.md)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

backend/app/models/__init__.py

@@ -17,6 +17,7 @@ from .invoice import Invoice, InvoiceType, InvoiceStatus
 from .referral import ReferralCode, Referral
 from .search_provider import SearchProvider
 from .discovery_record import DiscoveryRecord
+from .ai_provider import AIProvider
 
 __all__ = [
     "User", "Product",
@@ -38,4 +39,5 @@ __all__ = [
     "ReferralCode", "Referral",
     "SearchProvider",
     "DiscoveryRecord",
+    "AIProvider",
 ]