fix: add missing created_at and deleted_at columns to notification_preferences

- Remove duplicate updated_at from NotificationPreference model (inherited from TimestampMixin)
- Add migration 0018 to add created_at and deleted_at columns
- Fixes 500 error on GET /notifications/types and /preferences

alembic/versions/0018_fix_notification_preferences_columns.py

@@ -0,0 +1,30 @@
+"""Fix notification_preferences table: add missing created_at and deleted_at columns.
+
+Revision ID: 0018
+Revises: 0017
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0018"
+down_revision = "0017"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Add missing columns from TimestampMixin and SoftDeleteMixin
+    op.add_column(
+        "notification_preferences",
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False, server_default=sa.func.now()),
+    )
+    op.add_column(
+        "notification_preferences",
+        sa.Column("deleted_at", sa.DateTime(timezone=True), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("notification_preferences", "deleted_at")
+    op.drop_column("notification_preferences", "created_at")