feat: Add light/dark mode support with backend persistence

backend/alembic/versions/2473a00afb70_add_theme_preference_to_user.py

@@ -0,0 +1,28 @@
+"""Add theme preference to user
+
+Revision ID: 2473a00afb70
+Revises: 63886baa1255
+Create Date: 2026-02-16 17:00:25.537643
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '2473a00afb70'
+down_revision: Union[str, Sequence[str], None] = '63886baa1255'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    op.add_column('users', sa.Column('theme_preference', sa.String(), nullable=False, server_default='light'))
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.drop_column('users', 'theme_preference')