Adds `AppMembershipDetail` (membership row + joined username, email,
instance_role, is_active) and `list_for_app_enriched` on
`AppMembersRepository`. The Postgres impl does a single JOIN on
admin_users ordered by username, so the upcoming `GET
/apps/{id}/members` handler can render its table without an N+1 fetch.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>