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/post/admin/routes.py

@@ -562,6 +562,7 @@ def register():
     elif status and status != current_status:
       effective_status = status
 
+    sx_content_raw = form.get("sx_content", "").strip() or None
     try:
       post = await writer_update(
           g.s,
@@ -573,6 +574,7 @@ def register():
           custom_excerpt=custom_excerpt or None,
           feature_image_caption=feature_image_caption or None,
           status=effective_status,
+          sx_content=sx_content_raw,
       )
     except OptimisticLockError:
       return redirect(