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>

events/bp/calendar/routes.py

@@ -235,7 +235,7 @@ def register():
                 )
             ).scalars().all()
 
-            associated_entries = await get_associated_entries(g.s, post_id)
+            associated_entries = await get_associated_entries(post_id)
             nav_oob = render_post_nav_entries_oob(associated_entries, cals, post_data["post"])
             html = html + nav_oob
 

events/bp/calendar_entry/routes.py

@@ -135,7 +135,7 @@ def register():
         for post in entry_posts:
             # Get associated entries for this post
             from shared.services.entry_associations import get_associated_entries
-            associated_entries = await get_associated_entries(g.s, post.id)
+            associated_entries = await get_associated_entries(post.id)
 
             # Load calendars for this post
             from models.calendars import Calendar

events/bp/calendars/routes.py

@@ -84,7 +84,7 @@ def register():
                 )
             ).scalars().all()
 
-            associated_entries = await get_associated_entries(g.s, post_id)
+            associated_entries = await get_associated_entries(post_id)
             nav_oob = render_post_nav_entries_oob(associated_entries, cals, post_data["post"])
             html = html + nav_oob