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>

blog/bp/post/services/markets.py

@@ -7,7 +7,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from shared.contracts.dtos import MarketPlaceDTO
 from shared.infrastructure.actions import call_action, ActionError
-from shared.services.registry import services
+from services import blog_service
 
 
 class MarketError(ValueError):
@@ -33,7 +33,7 @@ async def create_market(sess: AsyncSession, post_id: int, name: str) -> MarketPl
         raise MarketError("Market name must not be empty.")
     slug = slugify(name)
 
-    post = await services.blog.get_post_by_id(sess, post_id)
+    post = await blog_service.get_post_by_id(sess, post_id)
     if not post:
         raise MarketError(f"Post {post_id} does not exist.")
 
@@ -57,7 +57,7 @@ async def create_market(sess: AsyncSession, post_id: int, name: str) -> MarketPl
 
 
 async def soft_delete_market(sess: AsyncSession, post_slug: str, market_slug: str) -> bool:
-    post = await services.blog.get_post_by_slug(sess, post_slug)
+    post = await blog_service.get_post_by_slug(sess, post_slug)
     if not post:
         return False