fix: make migration 0020 idempotent (check column exists)

alembic/versions/0020_notifications_updated_at.py

@@ -15,6 +15,14 @@ depends_on = None
 
 
 def upgrade() -> None:
+    conn = op.get_bind()
+    result = conn.execute(
+        sa.text(
+            "SELECT column_name FROM information_schema.columns "
+            "WHERE table_name = 'notifications' AND column_name = 'updated_at'"
+        )
+    )
+    if result.fetchone() is None:
         op.add_column(
             "notifications",
             sa.Column(