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/blog/ghost/ghost_sync.py

@@ -13,7 +13,7 @@ from sqlalchemy.orm.attributes import flag_modified  # for non-Mutable JSON colu
 from models.ghost_content import (
     Post, Author, Tag, PostAuthor, PostTag
 )
-from cart.models.page_config import PageConfig
+from shared.models.page_config import PageConfig
 
 # User-centric membership models
 from shared.models import User