Pentest R86: Vertrags-Identifier max 100 + Charset-Whitelist

R86.1 LOW + R86.2 LOW: >999-Zeichen liefen in DB-Overflow (500
statt 400), Attribut-Injection (`foo" onerror=…` ohne
umschließenden Tag) überlebte stripHtml.

Fix: validateContractIdentifier() (max 100,
^[A-Za-z0-9_\-/. ]{0,100}$) in sanitize.ts, eingehängt in
sanitizeContractBody. Wirft ApiError(400, …). Literales Space
statt \s → kein CRLF/Tab → kein Header-Injection-Vektor in
CSV-/Mail-/PDF-Export. Greift auf alle fünf Identifier-Felder
(Provider + Sales-Platform). ContractForm-Inputs bekommen
maxLength={100} als UX-Schicht.

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

backend/src/controllers/contract.controller.ts

@@ -8,7 +8,7 @@ import * as authorizationService from '../services/authorization.service.js';
 import { recordPredecessorFinalReading } from '../services/customer.service.js';
 import { ApiResponse, AuthRequest } from '../types/index.js';
 import { logChange } from '../services/audit.service.js';
-import { sanitizeContract, sanitizeContractStrict, sanitizeContracts, sanitizeContractsStrict, stripHtml, sanitizeNotes, validateContractDocumentType, validateOptionalIsoDate } from '../utils/sanitize.js';
+import { sanitizeContract, sanitizeContractStrict, sanitizeContracts, sanitizeContractsStrict, stripHtml, sanitizeNotes, validateContractDocumentType, validateOptionalIsoDate, isContractIdentifierField, validateContractIdentifier } from '../utils/sanitize.js';
 import { ApiError } from '../utils/apiError.js';
 import { canAccessContract } from '../utils/accessControl.js';
 import { maybeActivateOnDeliveryConfirmation, withContractDocumentLock } from '../services/contractStatusScheduler.service.js';
@@ -36,7 +36,14 @@ function sanitizeContractBody(body: unknown, parentKey?: string): unknown {
   if (body === null || body === undefined) return body;
   if (typeof body === 'string') {
     if (parentKey && PASSTHROUGH_KEYS.has(parentKey)) return body;
-    return stripHtml(body);
+    const stripped = stripHtml(body);
+    // Pentest 86.1/86.2 (LOW, 2026-06-19): Längen- + Whitelist-Check auf
+    // Kunden-/Vertrags-/Auftragsnummer-Feldern. validateContractIdentifier
+    // wirft ApiError(400) bei Verstoß → saubere 400-Antwort statt 500.
+    if (parentKey && isContractIdentifierField(parentKey)) {
+      return validateContractIdentifier(stripped, parentKey);
+    }
+    return stripped;
   }
   if (Array.isArray(body)) return body.map((v) => sanitizeContractBody(v, parentKey));
   if (typeof body === 'object') {