Add a reminder to many contacts (v2)

Create one reminder per contact for up to 500 contacts (contact_ids; duplicates are dropped), each linked to its one contact and sharing reminder's text, due_at_date, due_at_time and assignee_id (omitted = the caller, null = unassigned, otherwise an active workspace member), so completing one leaves the others open. All or nothing: a contact outside the workspace or an invalid assignee is a 400 invalid_reference and creates no reminder. The assignee is notified once for the whole batch. Viewers are refused. Workspace-only: send the X-Dex-Workspace header (400 workspace_context_required without it). Requires an Idempotency-Key header. Returns { data: { reminders: [...] } } in contact_ids order with status 201.