feat(cmp): Phase 3 — backend consent withdrawal + consent_id tracking

- ConsentBanner: save consent_id to localStorage after successful POST
- Footer: DELETE /api/consent/{id} on consent re-open (Art. 17 DSGVO)
- New proxy route: DELETE /api/consent/[id] → backend withdrawal endpoint

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

marketing-website/components/layout/ConsentBanner.tsx

@@ -127,12 +127,12 @@ function detectCookies(): CookieEntry[] {
   return cookies
 }
 
-async function sendConsent(consent: ConsentState, method: ConsentMethod, vendorConsents?: Record<string, boolean>) {
+async function sendConsent(consent: ConsentState, method: ConsentMethod, vendorConsents?: Record<string, boolean>): Promise<string | null> {
   try {
     const { device_type, browser, os } = detectDevice()
     const { blocked, released } = detectScripts()
     const cookies_set = detectCookies()
-    await fetch('/api/consent', {
+    const res = await fetch('/api/consent', {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify({
@@ -161,8 +161,14 @@ async function sendConsent(consent: ConsentState, method: ConsentMethod, vendorC
         cookies_set,
       }),
     })
+    const result = await res.json().catch(() => null)
+    if (result?.id) {
+      localStorage.setItem('bp_consent_id', result.id)
+      return result.id
+    }
+    return null
   } catch {
-    // Consent API not reachable — store locally anyway
+    return null
   }
 }
 

marketing-website/components/layout/Footer.tsx

@@ -4,7 +4,12 @@ import { Cookie } from 'lucide-react'
 import { t } from '@/lib/content'
 import { useApp } from '@/lib/context'
 
-function reopenConsentBanner() {
+async function reopenConsentBanner() {
+  const consentId = localStorage.getItem('bp_consent_id')
+  if (consentId) {
+    fetch(`/api/consent/${consentId}`, { method: 'DELETE' }).catch(() => {})
+    localStorage.removeItem('bp_consent_id')
+  }
   localStorage.removeItem('bp_consent')
   window.location.reload()
 }