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/admin/routes.py

@@ -264,7 +264,7 @@ def register():
 
     # Get associated entry IDs for this post
     post_id = g.post_data["post"]["id"]
-    associated_entry_ids = await get_post_entry_ids(g.s, post_id)
+    associated_entry_ids = await get_post_entry_ids(post_id)
 
     html = await render_template(
       "_types/post/admin/_calendar_view.html",
@@ -293,7 +293,7 @@ def register():
     from sqlalchemy import select
 
     post_id = g.post_data["post"]["id"]
-    associated_entry_ids = await get_post_entry_ids(g.s, post_id)
+    associated_entry_ids = await get_post_entry_ids(post_id)
 
     # Load ALL calendars (not just this post's calendars)
     result = await g.s.execute(
@@ -332,7 +332,7 @@ def register():
     from quart import jsonify
 
     post_id = g.post_data["post"]["id"]
-    is_associated, error = await toggle_entry_association(g.s, post_id, entry_id)
+    is_associated, error = await toggle_entry_association(post_id, entry_id)
 
     if error:
       return jsonify({"message": error, "errors": {}}), 400
@@ -340,7 +340,7 @@ def register():
     await g.s.flush()
 
     # Return updated association status
-    associated_entry_ids = await get_post_entry_ids(g.s, post_id)
+    associated_entry_ids = await get_post_entry_ids(post_id)
 
     # Load ALL calendars
     result = await g.s.execute(
@@ -355,7 +355,7 @@ def register():
       await g.s.refresh(calendar, ["entries", "post"])
 
     # Fetch associated entries for nav display
-    associated_entries = await get_associated_entries(g.s, post_id)
+    associated_entries = await get_associated_entries(post_id)
 
     # Load calendars for this post (for nav display)
     calendars = (