Decouple events: use shared.models for all cross-app imports

- Replace all imports from blog.models and cart.models
  with shared.models equivalents
- Convert events/models/calendars.py to re-export stub
- Update shared + glue submodule pointers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bp/payments/routes.py

@@ -5,7 +5,7 @@ from quart import (
 )
 from sqlalchemy import select
 
-from cart.models.page_config import PageConfig
+from shared.models.page_config import PageConfig
 
 from shared.browser.app.authz import require_admin
 from shared.browser.app.utils.htmx import is_htmx_request