Add SX block editor with Koenig-quality controls and lexical-to-sx converter

Pure s-expression block editor replacing React/Koenig: single hover + button,
slash commands, full card edit modes (image/gallery/video/audio/file/embed/
bookmark/callout/toggle/button/HTML/code), inline format toolbar, keyboard
shortcuts, drag-drop uploads, oEmbed/bookmark metadata fetching.

Includes lexical_to_sx converter for backfilling existing posts, KG card
components matching Ghost's card CSS, migration for sx_content column, and
31 converter tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blog/scripts/backfill_sx_content.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+"""
+Backfill sx_content from lexical JSON for all posts that have lexical but no sx_content.
+
+Usage:
+    python -m blog.scripts.backfill_sx_content [--dry-run]
+"""
+from __future__ import annotations
+
+import argparse
+import asyncio
+import sys
+
+from sqlalchemy import select, and_
+from sqlalchemy.ext.asyncio import AsyncSession
+
+
+async def backfill(dry_run: bool = False) -> int:
+    from shared.db.sessions import get_session_factory
+    from blog.models.content import Post
+    from blog.bp.blog.ghost.lexical_to_sx import lexical_to_sx
+
+    session_factory = get_session_factory("blog")
+    converted = 0
+    errors = 0
+
+    async with session_factory() as sess:
+        stmt = select(Post).where(
+            and_(
+                Post.lexical.isnot(None),
+                Post.lexical != "",
+                (Post.sx_content.is_(None)) | (Post.sx_content == ""),
+            )
+        )
+        result = await sess.execute(stmt)
+        posts = result.scalars().all()
+
+        print(f"Found {len(posts)} posts to convert")
+
+        for post in posts:
+            try:
+                sx = lexical_to_sx(post.lexical)
+                if dry_run:
+                    print(f"  [DRY RUN] {post.slug}: {len(sx)} chars")
+                else:
+                    post.sx_content = sx
+                    print(f"  Converted: {post.slug} ({len(sx)} chars)")
+                converted += 1
+            except Exception as e:
+                print(f"  ERROR: {post.slug}: {e}", file=sys.stderr)
+                errors += 1
+
+        if not dry_run:
+            await sess.commit()
+
+    print(f"\nDone: {converted} converted, {errors} errors")
+    return converted
+
+
+def main():
+    parser = argparse.ArgumentParser(description="Backfill sx_content from lexical JSON")
+    parser.add_argument("--dry-run", action="store_true", help="Don't write to database")
+    args = parser.parse_args()
+
+    asyncio.run(backfill(dry_run=args.dry_run))
+
+
+if __name__ == "__main__":
+    main()