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>

plugins/wordpress_advanced/plugin.py

@@ -67,7 +67,7 @@ class WordPressAdvancedPlugin(BasePlugin):
         if not container_name:
             raise ValueError(
                 "WordPress Advanced plugin requires 'container' configuration. "
-                "Please set WORDPRESS_ADVANCED_SITE1_CONTAINER in environment variables."
+                "Please set the 'container' field when adding the site in the dashboard."
             )
 
         # Import WP-CLI manager