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

- Replace blog.models import with shared.models equivalent
- Convert market/models/market.py and market_place.py to re-export stubs
- Update shared + glue submodule pointers

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

app.py

@@ -55,7 +55,7 @@ async def market_context() -> dict:
 
 def create_app() -> "Quart":
     from models.market_place import MarketPlace
-    from blog.models.ghost_content import Post
+    from shared.models.ghost_content import Post
 
     app = create_base_app("market", context_fn=market_context)