registry: default database.enabled to "prefer"

What does this MR do and why?

Changes the default value of registry['database']['enabled'] from "false" to "prefer".

With "prefer" mode, fresh installations using GitLab-managed PostgreSQL automatically enable the metadata database. Existing deployments using external PostgreSQL are unaffected — parse_database_configuration already overrides "prefer" to "false" when embedded PostgreSQL is disabled, preserving backward compatibility.

MR acceptance checklist

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Closes #9396 (closed)