Records the actual signature for whichever signer currently has status AWAIT_SIGNATURE — despite the generic name, only sign_url is settable (you cannot rename/retype a signer here). CUSTOMER signatures are rejected here (they flow through the customer portal instead). For an INTERNAL signer, the authenticated user must be that exact signer (self-sign only). When the last PENDING signer finishes and every signature is SIGNED, the estimate auto-transitions to APPROVED (stamps accepted_date, pushes a status_history entry, and triggers post-approval processing). Otherwise the next PENDING signer is advanced to AWAIT_SIGNATURE and notified (email + push).