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>

shared/contracts/dtos.py

@@ -87,6 +87,7 @@ class PostDTO:
     is_page: bool = False
     feature_image: str | None = None
     html: str | None = None
+    sx_content: str | None = None
     excerpt: str | None = None
     custom_excerpt: str | None = None
     published_at: datetime | None = None