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- docs/PROJECT_STATUS.md: archive (fully superseded by PROGRESS.md)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

@@ -3,6 +3,7 @@ from sqlalchemy import select, func
 from fastapi import HTTPException, Depends
 from datetime import datetime, date
 from sqlalchemy import Date
+from typing import Tuple
 import logging
 
 from app.models import UsageLog, SystemConfig, User, Customer, Product
@@ -75,7 +76,7 @@ class UsageService:
         result = await self.db.execute(stmt)
         return result.scalar() or 0
 
-    async def check_quota(self, user_id: str, action: str, chars: int = 0) -> tuple[bool, str]:
+    async def check_quota(self, user_id: str, action: str, chars: int = 0) -> Tuple[bool, str]:
         tier = await self.get_tier(user_id)
         limits = await self.get_limits(tier)
         limit_key = ACTION_MAP.get(action)