feat: add scratchpad column to sessions table

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/alembic/versions/009_add_scratchpad_to_sessions.py

@@ -0,0 +1,30 @@
+"""add scratchpad to sessions
+
+Revision ID: 009
+Revises: 4cdb5cba1aff
+Create Date: 2026-02-04
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '009'
+down_revision: Union[str, None] = '4cdb5cba1aff'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('sessions',
+        sa.Column('scratchpad', sa.Text(), nullable=True, server_default=sa.text("''"))
+    )
+    # Backfill existing rows to empty string (not NULL)
+    op.execute("UPDATE sessions SET scratchpad = '' WHERE scratchpad IS NULL")
+
+
+def downgrade() -> None:
+    op.drop_column('sessions', 'scratchpad')