perf: run bcrypt on a thread so it doesn't block the event loop

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).

tests/test_auth_async.py

@@ -0,0 +1,51 @@
+"""
+averify_password / ahash_password run bcrypt on a thread so the event
+loop can serve other requests while hashing. Contract: they must produce
+identical results to the sync versions.
+"""
+import pytest
+
+from decnet.web.auth import (
+    ahash_password,
+    averify_password,
+    get_password_hash,
+    verify_password,
+)
+
+
+@pytest.mark.asyncio
+async def test_ahash_matches_sync_hash_verify():
+    hashed = await ahash_password("hunter2")
+    assert verify_password("hunter2", hashed)
+    assert not verify_password("wrong", hashed)
+
+
+@pytest.mark.asyncio
+async def test_averify_matches_sync_verify():
+    hashed = get_password_hash("s3cret")
+    assert await averify_password("s3cret", hashed) is True
+    assert await averify_password("s3cre", hashed) is False
+
+
+@pytest.mark.asyncio
+async def test_averify_does_not_block_loop():
+    """Two concurrent averify calls should run in parallel (on threads).
+
+    With `asyncio.to_thread`, total wall time is ~max(a, b), not a+b.
+    """
+    import asyncio, time
+
+    hashed = get_password_hash("x")
+    t0 = time.perf_counter()
+    a, b = await asyncio.gather(
+        averify_password("x", hashed),
+        averify_password("x", hashed),
+    )
+    elapsed = time.perf_counter() - t0
+    assert a and b
+    # Sequential would be ~2× a single verify. Parallel on threads is ~1×.
+    # Single verify is ~250ms at rounds=12. Allow slack for CI noise.
+    single = time.perf_counter()
+    verify_password("x", hashed)
+    single_time = time.perf_counter() - single
+    assert elapsed < 1.7 * single_time, f"concurrent {elapsed:.3f}s vs single {single_time:.3f}s"