Wire sx_content through full read/write pipeline

Model: add sx_content column to Post. Writer: accept sx_content in
create_post, create_page, update_post. Routes: read sx_content from form
data in new post, new page, and edit routes. Read pipeline: ghost_db
includes sx_content in public dict, detail/home views prefer sx_content
over html when available, PostDTO includes sx_content.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blog/services/__init__.py

@@ -19,6 +19,7 @@ def _post_to_dto(post: Post) -> PostDTO:
         is_page=post.is_page,
         feature_image=post.feature_image,
         html=post.html,
+        sx_content=post.sx_content,
         excerpt=post.excerpt,
         custom_excerpt=post.custom_excerpt,
         published_at=post.published_at,