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/calendars/services/calendars.py

@@ -4,7 +4,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from models.calendars import Calendar
-from blog.models.ghost_content import Post  # for FK existence checks
+from shared.models.ghost_content import Post  # for FK existence checks
 from glue.services.relationships import attach_child, detach_child
 import unicodedata
 import re