Adds /api/v1/admin/users list / DELETE / PATCH guarded by RequireAdmin,
plus the audit-log substrate every future destructive admin endpoint
will reuse.
Safety properties:
- Cannot self-delete or self-demote (409 conflict, message calls out
"yourself" so the UI can render an explanation).
- Cannot remove the last admin via either DELETE or demote. The check
takes pg_advisory_xact_lock(ADMIN_INVARIANT_LOCK_KEY) and re-counts
admins inside the same tx, closing the parallel-demote race that a
bare "if count > 1" check would let through. The HTTP-serial path to
this guard is structurally unreachable (the actor would have to be
the lone admin demoting themselves, which the self-guard fires on
first); the parallel race test exercises it via repo calls.
Audit log (admin_audit table) records the action inside the same tx
as the action itself, so a rolled-back action never leaves an orphan
audit row. actor_user_id is ON DELETE SET NULL so the log outlives a
later-deleted admin. target_id is not a FK because future audit kinds
will target non-user rows.
0b2018ceca