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/alembic/versions/0005_add_sx_content.py

@@ -0,0 +1,20 @@
+"""Add sx_content column to posts table.
+
+Revision ID: blog_0005
+Revises: blog_0004
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "blog_0005"
+down_revision = "blog_0004"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column("posts", sa.Column("sx_content", sa.Text(), nullable=True))
+
+
+def downgrade():
+    op.drop_column("posts", "sx_content")