chore(F.4e): remove env-based site loading, unify OAuth Clients, update docs

- Remove env-based site discovery from SiteManager and ProjectManager
- Unify admin and user OAuth Clients into single role-based page
- Update all documentation to reflect DB-based site management
- Clean up env.example, README, ARCHITECTURE docs
- Improve Projects page empty state with link to My Sites
- Remove obsolete env discovery tests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/api_keys.py

@@ -10,12 +10,12 @@ import json
 import logging
 import os
 import secrets
+
+import bcrypt
 from dataclasses import asdict, dataclass
 from datetime import datetime, timedelta
 from pathlib import Path
 
-import bcrypt
-
 logger = logging.getLogger(__name__)