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/alembic/env.py

@@ -2,7 +2,7 @@ from alembic import context
 from shared.db.alembic_env import run_alembic
 
 MODELS = [
-    "shared.models.ghost_content",
+    "blog.models.content",
     "shared.models.kv",
     "shared.models.menu_item",
     "shared.models.menu_node",