Adds CannedResponse and AccountUser to the per-account cacheable_models so
their cache keys are exposed via /cache_keys and broadcast over ActionCable
on mutation. The endpoint now delegates to Account#cache_keys instead of
listing the three models inline, so future additions only need to touch the
concern.
AccountUser uses bespoke field-gated callbacks rather than the blanket
AccountCacheRevalidator: active_at is bumped on every page navigation and
would otherwise invalidate the agent cache constantly. User adds a
cross-account callback so name/email/display_name/confirmed_at changes
propagate, since those fields appear in the agent serializer but live on
the User model.
Shivam Mishra
991bc5a265