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/export.py

@@ -4,8 +4,8 @@ Uses REST APIs:
 - Export API (GET /export/events, /export/charts) for raw data export
 - Insights API (GET /insights/:projectId/*) for analytics queries
 
-Note: project_id is optional if configured in environment variables.
-When not provided, the default project_id from OPENPANEL_SITE1_PROJECT_ID is used.
+Note: project_id is optional if configured in the site settings.
+When not provided, the default project_id from the site configuration is used.
 
 Requires 'read' or 'root' mode client for Export API.
 """

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."
     )