Enable warming (async)

Asynchronously enable warming for email users. Creates a background job that processes mailboxes via SQS. Each mailbox is enabled independently with automatic retries on failure. Track progress via the returned job_id using GET /v1/jobs/{job_id} or a Firestore listener on the job document. Warming is billed at enable time, before the job is created (see /v1/warming/enable); a payment failure returns 402 and no job is created.