Idempotency

Make a retried send return the original lead for 24 hours.

Send Idempotency-Key on POST /api/leads to make a retry safe for 24 hours. The key is scoped to this account and agreementId.

The same key, phone, and email returns the original 201 and the original id. It does not create a second lead. A declined lead still replays that 201. GET /api/leads/{id} after decline is 404.

The same key with a different phone or email is 409 and creates nothing.

No Idempotency-Key header means every call creates a lead.

A second call while the first is still running is 503 and creates nothing.