Connection and tuning settings for Gamend.Repo.
The adapter is chosen at compile time (see config/host_config.exs), so
adapter here is documentation and admin display rather than something a
restart can change — the app refuses to start on a stale build and says so.
GAMEND_DB_URL and the POSTGRES_* family are inherited names: platforms
provision them, and renaming them would break every managed-database
attachment.