DeviceUpdate accepts user_list, so a device PUT could add or remove users
without touching their device_serials - leaving the MQTT app ACL stale
(a removed user would keep access; an added user would be denied).
update_device now diffs the old vs new user_list and, in the same atomic
batch as the device write, ArrayUnion/ArrayRemoves the device's serial on
each added/removed user, then invalidates their MQTT ACL cache entries.
Dangling user references are skipped (updating a missing doc would fail
the whole batch). PUTs without user_list take the old single-update path.
Covered by tests/test_device_serials_sync.py (fake Firestore).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
