verify_password / get_password_hash are CPU-bound and take ~250ms each
at rounds=12. Called directly from async endpoints, they stall every
other coroutine for that window — the single biggest single-worker
bottleneck on the login path.
Adds averify_password / ahash_password that wrap the sync versions in
asyncio.to_thread. Sync versions stay put because _ensure_admin_user and
tests still use them.
5 call sites updated: login, change-password, create-user, reset-password.
tests/test_auth_async.py asserts parallel averify runs concurrently (~1x
of a single verify, not 2x).
