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/openpanel/handlers/utils.py

@@ -23,6 +23,6 @@ def get_project_id(client: OpenPanelClient, project_id: str | None) -> str:
         return client.default_project_id
     raise ValueError(
         "project_id is required. Either provide it as a parameter or configure "
-        "OPENPANEL_SITE1_PROJECT_ID in environment variables. "
+        "the project_id field when adding the site in the dashboard. "
         "You can find your Project ID in OpenPanel Dashboard → Project Settings."
     )