Record a sign-up at the rep's shift with the customer's signed consent (retry-safe on client_capture_id)

Body: client_capture_id, shift_id, first_name, last_name, phone, email, note, disclosure_version_id, signer_name, agreed (true), signed_at, location {latitude, longitude, accuracy_meters, captured_at}, device. 403 not_assigned, 409 outside_shift_window / disclosure_not_found. The reply never says whether the number already belonged to a contact. No text message is sent.