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- docs/PROJECT_STATUS.md: archive (fully superseded by PROGRESS.md)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

@@ -0,0 +1,22 @@
+from sqlalchemy import Column, String, Integer, DateTime, Boolean, Text
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+from datetime import datetime
+from app.database import Base
+import uuid
+
+
+class AIProvider(Base):
+    __tablename__ = "ai_providers"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name = Column(String(100), nullable=False)
+    provider_type = Column(String(50), nullable=False)
+    api_key = Column(Text, nullable=True)
+    api_secret = Column(Text, nullable=True)
+    base_url = Column(String(500), nullable=True)
+    model_name = Column(String(100), nullable=False)
+    extra_config = Column(JSONB, default={})
+    priority = Column(Integer, default=0)
+    enabled = Column(Boolean, default=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)