Adds a signer to a proposal's multi-signer flow (up to 5 signers: max 1 CUSTOMER, 1 INTERNAL, 3 COSIGNER — a CUSTOMER signer is auto-created if none exists). Requires the multi_signer company policy to be enabled, the estimate to be a proposal (is_proposal: true) with a layout association, and estimate_status to be DRAFT/AWAIT_RESPONSE/AWAIT_SIGNATURE. type defaults to COSIGNER; CUSTOMER cannot be added here. COSIGNER requires email+name; INTERNAL requires user_uid (email/name are overwritten from the resolved user). If the immediately-preceding signer has already signed, the new signer starts in AWAIT_SIGNATURE (gets notified immediately) instead of PENDING.