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/fragments/routes.py

@@ -125,7 +125,7 @@ def register():
                          data_app="blog")
 
     async def _link_card_handler():
-        from shared.services.registry import services
+        from services import blog_service
         from shared.infrastructure.urls import blog_url
 
         slug = request.args.get("slug", "")
@@ -137,7 +137,7 @@ def register():
             parts = []
             for s in slugs:
                 parts.append(f"<!-- fragment:{s} -->")
-                post = await services.blog.get_post_by_slug(g.s, s)
+                post = await blog_service.get_post_by_slug(g.s, s)
                 if post:
                     parts.append(_blog_link_card_sx(post, blog_url(f"/{post.slug}")))
             return "\n".join(parts)
@@ -145,7 +145,7 @@ def register():
         # Single mode
         if not slug:
             return ""
-        post = await services.blog.get_post_by_slug(g.s, slug)
+        post = await blog_service.get_post_by_slug(g.s, slug)
         if not post:
             return ""
         return _blog_link_card_sx(post, blog_url(f"/{post.slug}"))