Pentest R110: Mass-Assignment-Whitelist auf 7 Katalog-Endpunkten

MEDIUM: PUT /api/stressfrei-emails/:id und 6 weitere Update-
Endpunkte (platform, tariff, contractCategory, cancellationPeriod,
contractDuration, email-providers) reichten req.body ungefiltert
an Prisma. Gleiche Bug-Klasse wie das gefixte M1-Finding, sieben
Stellen mehr. Nachgewiesen via provisionError-Feld ausserhalb des
TS-Types.

Fix: sieben Whitelists + pickXxxUpdate()-Helper in sanitize.ts,
in den jeweiligen Controllern eingehängt. Reuse der bewährten
pick()-Infrastruktur (Customer/User seit Runde 7).

EmailProvider bewusst OHNE stripHtmlFromStrings, weil Passwörter
und API-Keys legitim Sonderzeichen enthalten dürfen.

Doku: SECURITY-HARDENING.md § Runde 110 + docs/todo.md.

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

backend/src/controllers/contract-duration.controller.ts

@@ -2,6 +2,7 @@ import { Request, Response } from 'express';
 import * as contractDurationService from '../services/contract-duration.service.js';
 import { logChange } from '../services/audit.service.js';
 import { ApiResponse } from '../types/index.js';
+import { pickContractDurationUpdate } from '../utils/sanitize.js';
 
 export async function getContractDurations(req: Request, res: Response): Promise<void> {
   try {
@@ -54,7 +55,8 @@ export async function createContractDuration(req: Request, res: Response): Promi
 
 export async function updateContractDuration(req: Request, res: Response): Promise<void> {
   try {
-    const duration = await contractDurationService.updateContractDuration(parseInt(req.params.id), req.body);
+    // Pentest R110: Mass-Assignment-Whitelist.
+    const duration = await contractDurationService.updateContractDuration(parseInt(req.params.id), pickContractDurationUpdate(req.body));
     await logChange({
       req, action: 'UPDATE', resourceType: 'ContractDuration',
       resourceId: duration.id.toString(),