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/app.py

@@ -134,7 +134,7 @@ def create_app() -> "Quart":
     async def oembed():
         from urllib.parse import urlparse
         from quart import jsonify
-        from shared.services.registry import services
+        from services import blog_service
         from shared.infrastructure.urls import blog_url
         from shared.infrastructure.oembed import build_oembed_response
 
@@ -147,7 +147,7 @@ def create_app() -> "Quart":
         if not slug:
             return jsonify({"error": "could not extract slug"}), 404
 
-        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 jsonify({"error": "not found"}), 404