Sicherheit: HTTP-Security-Header und Origin-Prüfung für schreibende Routen

- X-Frame-Options/CSP frame-ancestors gegen Clickjacking,
  nosniff, Referrer-Policy, Permissions-Policy
- Origin-Check als CSRF-Defense-in-Depth für /api/admin/users
  und /logout (zusätzlich zu SameSite=Lax-Cookies und JSON-Content-Type)

app/api/admin/users/route.ts

@@ -1,12 +1,14 @@
 import { NextResponse } from "next/server";
 import { createClient } from "@/lib/supabase/server";
 import { createAdminClient } from "@/lib/supabase/admin";
+import { isSameOrigin } from "@/lib/origin";
 
 // Benutzerverwaltung (nur Admin). Läuft serverseitig mit dem
 // Service-Role-Key, da das Anlegen/Löschen von Auth-Benutzern
 // nicht über den anon key möglich ist.
 
-async function requireAdmin() {
+async function requireAdmin(request: Request) {
+  if (!isSameOrigin(request)) return null;
   const supabase = await createClient();
   const {
     data: { user },
@@ -24,7 +26,7 @@ async function requireAdmin() {
 }
 
 export async function POST(request: Request) {
-  const ctx = await requireAdmin();
+  const ctx = await requireAdmin(request);
   if (!ctx) return NextResponse.json({ error: "Nicht berechtigt." }, { status: 403 });
 
   const body = await request.json().catch(() => null);
@@ -59,7 +61,7 @@ export async function POST(request: Request) {
 }
 
 export async function PATCH(request: Request) {
-  const ctx = await requireAdmin();
+  const ctx = await requireAdmin(request);
   if (!ctx) return NextResponse.json({ error: "Nicht berechtigt." }, { status: 403 });
 
   const body = await request.json().catch(() => null);
@@ -98,7 +100,7 @@ export async function PATCH(request: Request) {
 }
 
 export async function DELETE(request: Request) {
-  const ctx = await requireAdmin();
+  const ctx = await requireAdmin(request);
   if (!ctx) return NextResponse.json({ error: "Nicht berechtigt." }, { status: 403 });
 
   const body = await request.json().catch(() => null);

app/logout/route.ts

@@ -1,7 +1,11 @@
 import { NextResponse } from "next/server";
 import { createClient } from "@/lib/supabase/server";
+import { isSameOrigin } from "@/lib/origin";
 
 export async function POST(request: Request) {
+  if (!isSameOrigin(request)) {
+    return NextResponse.json({ error: "Nicht erlaubt." }, { status: 403 });
+  }
   const supabase = await createClient();
   await supabase.auth.signOut();
   return NextResponse.redirect(new URL("/login", request.url), { status: 303 });