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,

blog/services/post_writer.py

@@ -207,6 +207,7 @@ async def create_post(
     feature_image_caption: str | None = None,
     tag_names: list[str] | None = None,
     is_page: bool = False,
+    sx_content: str | None = None,
 ) -> Post:
     """Create a new post or page directly in db_blog."""
     html, plaintext, reading_time = _render_and_extract(lexical_json)
@@ -217,6 +218,7 @@ async def create_post(
         title=title or "Untitled",
         slug=slug,
         lexical=lexical_json if isinstance(lexical_json, str) else json.dumps(lexical_json),
+        sx_content=sx_content,
         html=html,
         plaintext=plaintext,
         reading_time=reading_time,
@@ -281,6 +283,7 @@ async def create_page(
     custom_excerpt: str | None = None,
     feature_image_caption: str | None = None,
     tag_names: list[str] | None = None,
+    sx_content: str | None = None,
 ) -> Post:
     """Create a new page. Convenience wrapper around create_post."""
     return await create_post(
@@ -294,6 +297,7 @@ async def create_page(
         feature_image_caption=feature_image_caption,
         tag_names=tag_names,
         is_page=True,
+        sx_content=sx_content,
     )
 
 
@@ -308,6 +312,7 @@ async def update_post(
     custom_excerpt: str | None = ...,  # type: ignore[assignment]
     feature_image_caption: str | None = ...,  # type: ignore[assignment]
     status: str | None = None,
+    sx_content: str | None = ...,  # type: ignore[assignment]
 ) -> Post:
     """Update post content. Optimistic lock via expected_updated_at.
 
@@ -342,6 +347,9 @@ async def update_post(
     if title is not None:
         post.title = title
 
+    if sx_content is not _SENTINEL:
+        post.sx_content = sx_content
+
     if feature_image is not _SENTINEL:
         post.feature_image = feature_image
     if custom_excerpt is not _SENTINEL: