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>

.env.example

@@ -5,9 +5,12 @@
 # Version: 3.0.0
 # Last Updated: 2026-02-17
 #
-# This file contains all environment variables needed to run the MCP server.
+# This file contains environment variables needed to run the MCP server.
 # Copy this file to .env and fill in your actual values.
 #
+# SITES: Sites are managed via the web dashboard (DB-based), not env vars.
+# The plugin sections below are kept as credential reference only.
+#
 # SECURITY NOTE: Never commit .env file to version control!
 #
 # Multi-Endpoint Architecture (v3.0.0):