feat: Initial implementation of Annas Rechnungsmanager

Full-stack German accounting & invoice management web application:

- Multi-company management (Mandantenverwaltung) with full CRUD
- Invoice creation with dynamic line items and automatic tax calculation
- Sequential invoice numbering per company (RE-2024-001 format)
- §14 UStG compliant PDF invoice generation via @react-pdf/renderer
- Customer management (Kundenverwaltung) per company
- Tax reports: quarterly USt-Voranmeldung and monthly revenue overview
- Email/password authentication via NextAuth.js v5
- Responsive, modern UI with Tailwind CSS and custom shadcn/ui components
- Prisma v5 ORM with MySQL/MariaDB schema + demo seed data

Stack: Next.js 14 (App Router) · TypeScript · Prisma/MySQL · NextAuth.js

https://claude.ai/code/session_01FN53KKxo5ebrGwqFhxzkT9

src/app/api/invoices/[id]/route.ts

@@ -0,0 +1,57 @@
+import { NextRequest, NextResponse } from "next/server";
+import { auth } from "@/lib/auth";
+import prisma from "@/lib/prisma";
+import { InvoiceStatus } from "@prisma/client";
+import { z } from "zod";
+
+async function getInvoice(id: string, userId: string) {
+  return prisma.invoice.findFirst({
+    where: { id, company: { userId } },
+    include: {
+      items: { orderBy: { position: "asc" } },
+      customer: true,
+      company: true,
+    },
+  });
+}
+
+export async function GET(_req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  const session = await auth();
+  if (!session?.user?.id) return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  const { id } = await params;
+  const invoice = await getInvoice(id, session.user.id);
+  if (!invoice) return NextResponse.json({ error: "Not found" }, { status: 404 });
+  return NextResponse.json(invoice);
+}
+
+const statusSchema = z.object({
+  status: z.nativeEnum(InvoiceStatus),
+});
+
+export async function PATCH(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  const session = await auth();
+  if (!session?.user?.id) return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  const { id } = await params;
+  const invoice = await getInvoice(id, session.user.id);
+  if (!invoice) return NextResponse.json({ error: "Not found" }, { status: 404 });
+
+  const body = await req.json();
+  const parsed = statusSchema.safeParse(body);
+  if (!parsed.success) return NextResponse.json({ error: parsed.error.issues }, { status: 400 });
+
+  const updated = await prisma.invoice.update({
+    where: { id },
+    data: { status: parsed.data.status },
+  });
+  return NextResponse.json(updated);
+}
+
+export async function DELETE(_req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  const session = await auth();
+  if (!session?.user?.id) return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  const { id } = await params;
+  const invoice = await getInvoice(id, session.user.id);
+  if (!invoice) return NextResponse.json({ error: "Not found" }, { status: 404 });
+  await prisma.invoice.delete({ where: { id } });
+  return NextResponse.json({ ok: true });
+}