gate2: fix duplicate column/index in migrations for fresh DB installation

alembic/versions/0018_fix_notification_preferences_columns.py

@@ -15,14 +15,8 @@ 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),
-    )
+    op.execute("ALTER TABLE notification_preferences ADD COLUMN IF NOT EXISTS created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()")
+    op.execute("ALTER TABLE notification_preferences ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ")
 
 
 def downgrade() -> None: