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/contracts/protocols.py

@@ -11,7 +11,6 @@ from typing import Protocol, runtime_checkable
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from .dtos import (
-    PostDTO,
     CalendarDTO,
     CalendarEntryDTO,
     TicketDTO,
@@ -29,17 +28,6 @@ from .dtos import (
 )
 
 
-@runtime_checkable
-class BlogService(Protocol):
-    async def get_post_by_slug(self, session: AsyncSession, slug: str) -> PostDTO | None: ...
-    async def get_post_by_id(self, session: AsyncSession, id: int) -> PostDTO | None: ...
-    async def get_posts_by_ids(self, session: AsyncSession, ids: list[int]) -> list[PostDTO]: ...
-
-    async def search_posts(
-        self, session: AsyncSession, query: str, page: int = 1, per_page: int = 10,
-    ) -> tuple[list[PostDTO], int]: ...
-
-
 @runtime_checkable
 class CalendarService(Protocol):
     async def calendars_for_container(