Decouple blog models and BlogService from shared layer

Move Post/Author/Tag/PostAuthor/PostTag/PostUser models from
shared/models/ghost_content.py to blog/models/content.py so blog-domain
models no longer live in the shared layer. Replace the shared
SqlBlogService + BlogService protocol with a blog-local singleton
(blog_service), and switch entry_associations.py from direct DB access
to HTTP fetch_data("blog", "post-by-id") to respect the inter-service
boundary.

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

shared/models/__init__.py

@@ -10,7 +10,6 @@ from .ghost_membership_entities import (
     GhostNewsletter, UserNewsletter,
     GhostTier, GhostSubscription,
 )
-from .ghost_content import Tag, Post, Author, PostAuthor, PostTag
 from .page_config import PageConfig
 from .order import Order, OrderItem
 from .market import (