Add keep-in-touch cadences in bulk (v2)

Add the Team cadence (kind: 'team'; owners, admins and standard members) or the caller's own My cadence (kind: 'my'; every member, viewers included) on one frequency to up to 1000 contacts (contact_ids). A contact that already has a cadence of that kind is skipped and keeps it unchanged (skipped_existing_contact_ids); a missing, archived or other-workspace contact is skipped too (not_found_contact_ids). Due dates are computed by the server. 400 frequency_not_found; a viewer's Team batch is refused like the single create. The contacts are written in pages that commit independently, so a failed request may have added some cadences: retrying the same body is safe. Workspace-only: send the X-Dex-Workspace header (400 workspace_context_required without it). Requires an Idempotency-Key header. Returns { data: { cadences: [...], skipped_existing_contact_ids, not_found_contact_ids } } with status 201.