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>

tests/test_admin_system.py

@@ -1,5 +1,6 @@
 """Tests for Admin System Unification (F.4)."""
 
+
 from core.dashboard.auth import (
     DashboardSession,
     get_session_display_info,