Vertrag: Auftragsnummer Vertriebsplattform vor Kundennummer

Contract.orderNumberAtSalesPlatform (VARCHAR(191) NULL) mit
Migration 20260619100000_contract_order_number_at_sales_platform
(IF NOT EXISTS). Form-Input, Detail-Zeile mit Copy-Button,
Audit-Mapping, Renewal-Copy und XSS-Strip-Allowlist analog zu
den bestehenden Sales-Platform-Feldern.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/utils/sanitize.ts

@@ -75,6 +75,7 @@ const CONTRACT_DISPLAY_STRING_FIELDS = [
   'tariffName',
   'customerNumberAtProvider',
   'contractNumberAtProvider',
+  'orderNumberAtSalesPlatform',
   'customerNumberAtSalesPlatform',
   'contractNumberAtSalesPlatform',
   'portalUsername',