Withdraw an unsigned sent proposal

Firm-side proposals. Select the firm with x-organization-id. Reads require active staff membership and access to every linked company; prospects are visible to their creator or firm managers. Writes require Firm Owner/Admin and billing permission. OAuth requires the broad mcp grant. Saves are revision-checked and idempotent using a stable requestId. All amounts are integer USD cents. Historical service dates and separately priced backwork are supported; saves never send, sign, provision companies or charge. Invalid dates, overlapping backwork/ongoing coverage and deposits exceeding the fee are rejected.