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

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

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

bp/post/routes.py

@@ -11,8 +11,8 @@ from quart import (
 )
 from .services.post_data import post_data
 from .services.post_operations import toggle_post_like
-from events.models.calendars import Calendar
-from market.models.market_place import MarketPlace
+from shared.models.calendars import Calendar
+from shared.models.market_place import MarketPlace
 from sqlalchemy import select
 
 from shared.browser.app.redis_cacher import cache_page, clear_cache