fix(visitor_mailer): follow the host reassigned on an event (PPT-2375)

[chillfox]

Scenario 1 of PPT-2375: "the 'Booking Host Changed' notification should be sent to the original host. Instead the new host is receiving the Visitor Invite Confirmation email." — the mailer half.

The reassignment itself is PlaceOS/staff-api#383, which makes host on every signal mean who is hosting and reports the mailbox owner separately as organiser_email. With that, the mailer's existing logic already does the right thing; this adds the model field, locks the behaviour down with tests, and deals with the other half of QA's report.

The new host receiving a visitor invitation

They were not being treated as a host at all — they were a newly added attendee, and the signal still named the old host, so skip_host_email had nothing to match. The staff API change fixes that case, but it is the visible symptom of something broader: front ends mark every attendee of a meeting as an expected visitor, so colleagues are announced exactly like external guests and receive visitor invitations and QR codes.

skip_internal_domain_email (default false) drops anyone sharing the host's email domain, on invitations, check-in and induction notifications, and change notifications. It is the no-configuration counterpart to host_domain_filter, which needs an explicit domain list.

Left off by default: staff visiting another site are legitimate visitors and share the host's domain, so turning this on for every deployment would silently stop those emails. Worth enabling per system — including the dev server, where it is what stops QA seeing the invitation to the new host.

Emails aimed at the host (notify_checkin, notify_original_host) are unaffected — the filter tests the attendee's domain, not the recipient's.

Testing

4 new specs: host_domain_filter and skip_internal_domain_email suppressing a colleague's invitation and their change notification while an external visitor still gets both, the setting being off by default, and a reassignment signal notifying the previous host while naming the new host on the visitor's change email.

./harness report --basic-render --verbose --tests=1 drivers/place/visitor_mailer_spec.cr passes; crystal tool format and ameba clean.

Stacked on #620.