MigrateRatingLevels issued an ExecuteUpdate while the SELECT DISTINCT reader
was still open on the same connection. SQLite tolerates that, Npgsql does not,
so the AppInitialisation stage aborted and every PostgreSQL instance
crash-looped on startup.
- read the distinct ratings into a list before the update loop
- cover the migration against a real PostgreSQL, with NULL and empty ratings
unkin-agent