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/services/registry.py

@@ -8,15 +8,14 @@ Usage::
     from shared.services.registry import services
 
     # Register at app startup (own domain only)
-    services.blog = SqlBlogService()
+    services.calendar = SqlCalendarService()
 
     # Use locally within the owning app
-    post = await services.blog.get_post_by_slug(session, slug)
+    cals = await services.calendar.calendars_for_container(session, "page", page_id)
 """
 from __future__ import annotations
 
 from shared.contracts.protocols import (
-    BlogService,
     CalendarService,
     MarketService,
     CartService,
@@ -33,23 +32,11 @@ class _ServiceRegistry:
     """
 
     def __init__(self) -> None:
-        self._blog: BlogService | None = None
         self._calendar: CalendarService | None = None
         self._market: MarketService | None = None
         self._cart: CartService | None = None
         self._federation: FederationService | None = None
 
-    # -- blog -----------------------------------------------------------------
-    @property
-    def blog(self) -> BlogService:
-        if self._blog is None:
-            raise RuntimeError("BlogService not registered")
-        return self._blog
-
-    @blog.setter
-    def blog(self, impl: BlogService) -> None:
-        self._blog = impl
-
     # -- calendar -------------------------------------------------------------
     @property
     def calendar(self) -> CalendarService: