Domain isolation: replace cross-domain imports with service calls

Replace direct Post, MarketPlace, Calendar model queries and HTTP API
calls with typed service calls. Events registers all 4 services via
domain_services_fn with has() guards.

Key changes:
- app.py: use domain_services_fn, Post/Calendar/MarketPlace queries
  → services.blog/calendar/market, HTTP cart API → services.cart
- calendars/markets services: Post → services.blog
- post_associations: Post → services.blog, direct queries → services
- markets routes: remove unused MarketPlace import
- glue imports → shared imports throughout

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

bp/markets/routes.py

@@ -3,9 +3,6 @@ from __future__ import annotations
 from quart import (
     request, render_template, make_response, Blueprint, g
 )
-from sqlalchemy import select
-
-from shared.models.market_place import MarketPlace
 
 from .services.markets import (
     create_market as svc_create_market,