List schema migrations (Supabase-style)

Returns migration records from supabase_migrations.schema_migrations. If the table does not exist, returns an empty array with a message instead of an error.

curl -X GET "https://athena-cluster.com/schema/migrations" \  -H "X-Athena-Client: string"

{
  "migrations": [
    {
      "version": "string",
      "name": "string"
    }
  ],
  "message": "string"
}