Add a user_settings realtime channel so clients learn when a setting
changed on another device without polling. The channel is modeled on
user_state: non-admin subscribable, per-user addressed envelopes, null
snapshot.
SettingValuesHandler gains an EventsHub and publishes
user_settings.changed after every successful PUT and DELETE on
/settings/values/{key}. The payload carries only key, scope and
profile_id — never the value. Admins receive every user's user-scoped
events, so a value in the payload would leak private settings to
admins; interested clients re-fetch over the scoped REST API instead.
The payload is always non-empty because an empty Data falls back to a
null snapshot in the hub. A nil hub (tests) skips publishing.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
