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/app/api/consent/[id]/route.ts

@@ -0,0 +1,21 @@
+import { NextRequest, NextResponse } from 'next/server'
+
+const BACKEND_URL = process.env.CONSENT_BACKEND_URL || 'https://macmini:3007/api/sdk/v1/banner'
+const TENANT_ID = process.env.CONSENT_TENANT_ID || '9282a473-5c95-4b3a-bf78-0ecc0ec71d3e'
+
+export async function DELETE(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  try {
+    const { id } = await params
+    const res = await fetch(`${BACKEND_URL}/consent/${id}`, {
+      method: 'DELETE',
+      headers: { 'X-Tenant-ID': TENANT_ID },
+    })
+    const data = await res.text()
+    return new NextResponse(data, {
+      status: res.status,
+      headers: { 'Content-Type': 'application/json' },
+    })
+  } catch {
+    return NextResponse.json({ error: 'Consent service not reachable' }, { status: 503 })
+  }
+}