Get warmup stats for one mailbox

Warmup health for a single mailbox, in both directions: the sending side (volume, placement, reputation) and the incoming side (incoming_warmup_state, last_received_at, and a plain-language incoming_warmup_state_detail explaining any pause). Also returns placement_by_provider — where this mailbox's warmup emails landed over the last 14 days, split by the receiving provider (Google, Microsoft, other providers) — and spam_hold, a plain sentence explaining a reduced daily volume while the spam check holds it (null otherwise).

Required scope: warmup:read.