When an event has no inline customer_email/host_email but has customer
account(s) assigned (event_customer_assignments), the pre-event reminder now
sends to those registered customers instead of skipping with no_recipient.
Recipients sent to an assigned account are queued WITHOUT eventId so the
language resolver uses the customer's preferred_language (vs the event's
language for inline-email sends). Applies to both the flow path
(sendReminderForEvent) and the legacy pass. The gallery-ready mail deliberately
does NOT fall back to accounts — only the reminder does. Test covers the
no-inline-email + assigned-customer case.