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/bp/blog/ghost_db.py

@@ -63,6 +63,7 @@ def _post_to_public(p: Post) -> Dict[str, Any]:
         "slug": p.slug,
         "title": p.title,
         "html": p.html,
+        "sx_content": p.sx_content,
         "is_page": p.is_page,
         "excerpt": p.custom_excerpt or p.excerpt,
         "custom_excerpt": p.custom_excerpt,