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>

shared/services/entry_associations.py

@@ -4,16 +4,12 @@ Only uses HTTP-based fetch_data/call_action, no direct DB access.
 """
 from __future__ import annotations
 
-from sqlalchemy.ext.asyncio import AsyncSession
-
 from shared.infrastructure.actions import call_action, ActionError
 from shared.infrastructure.data_client import fetch_data
 from shared.contracts.dtos import CalendarEntryDTO, dto_from_dict
-from shared.services.registry import services
 
 
 async def toggle_entry_association(
-    session: AsyncSession,
     post_id: int,
     entry_id: int
 ) -> tuple[bool, str | None]:
@@ -21,7 +17,7 @@ async def toggle_entry_association(
     Toggle association between a post and calendar entry.
     Returns (is_now_associated, error_message).
     """
-    post = await services.blog.get_post_by_id(session, post_id)
+    post = await fetch_data("blog", "post-by-id", params={"id": post_id}, required=False)
     if not post:
         return False, "Post not found"
 
@@ -35,7 +31,6 @@ async def toggle_entry_association(
 
 
 async def get_post_entry_ids(
-    session: AsyncSession,
     post_id: int
 ) -> set[int]:
     """
@@ -49,7 +44,6 @@ async def get_post_entry_ids(
 
 
 async def get_associated_entries(
-    session: AsyncSession,
     post_id: int,
     page: int = 1,
     per_page: int = 10