fix: konvertera alla API route handlers till withAuth wrapper

Ersätter getAuthHeaders() + auth() standalone med withAuth() wrapper
i alla route handlers. Auth() standalone fungerar inte korrekt i
Next.js 16 + NextAuth beta.28 pga async cookies() kompatibilitet.
withAuth() använder auth() i wrapper-form sa att request.auth
populeras direkt av NextAuth.

Pavaerkade filer: 27 route handlers + ny lib/with-auth.ts

frontend/app/api/recipe-preview-proxy/route.ts

@@ -1,31 +1,21 @@
-import { NextRequest, NextResponse } from 'next/server';
-import { getAuthHeaders } from '../../../lib/auth-headers';
+import { NextResponse } from 'next/server';
+import { withAuth } from '../../../lib/with-auth';
 
 const API_BASE = process.env.NEXT_PUBLIC_API_URL_INTERNAL || 'http://recipe-api:8080';
 
-export async function GET(request: NextRequest) {
-  const authHeaders = await getAuthHeaders();
-  const id = request.nextUrl.searchParams.get('id');
+export const GET = withAuth(async (request, session) => {
+  const id = new URL(request.url).searchParams.get('id');
 
   if (!id) {
-    return NextResponse.json(
-      { error: 'Missing id parameter' },
-      { status: 400 }
-    );
+    return NextResponse.json({ error: 'Missing id parameter' }, { status: 400 });
   }
 
   const res = await fetch(`${API_BASE}/api/recipes/${id}/inventory-preview`, {
-    method: 'GET',
-    headers: { ...authHeaders },
+    headers: { Authorization: `Bearer ${session.accessToken}` },
     cache: 'no-store',
   });
 
   const text = await res.text();
-
-  return new NextResponse(text, {
-    status: res.status,
-    headers: {
-      'Content-Type': 'application/json',
-    },
-  });
+  return new NextResponse(text, { status: res.status, headers: { 'Content-Type': 'application/json' } });
+});
 }