Update Content model to enforce status choices and add migration for status field changes

- Introduced STATUS_CHOICES in the Content model to restrict the status field to 'draft', 'review', and 'published'.
- Created a new migration to reflect the updated status choices without altering existing data.
- Removed 'completed' status from the frontend status color mapping for consistency.

frontend/src/pages/Writer/Content.tsx

@@ -9,7 +9,6 @@ const statusColors: Record<string, 'warning' | 'info' | 'success' | 'primary'> =
   draft: 'warning',
   review: 'info',
   published: 'success',
-  completed: 'success',
 };
 
 export default function Content() {