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/route.ts

@@ -0,0 +1,74 @@
+import { NextRequest, NextResponse } from "next/server";
+import { auth } from "@/lib/auth";
+import prisma from "@/lib/prisma";
+import { generateInvoiceNumber } from "@/lib/invoice-number";
+import { z } from "zod";
+
+const itemSchema = z.object({
+  position: z.number().int(),
+  description: z.string().min(1),
+  quantity: z.number().positive(),
+  unit: z.string().optional(),
+  unitPrice: z.number(),
+  taxRate: z.number(),
+  netAmount: z.number(),
+  taxAmount: z.number(),
+  grossAmount: z.number(),
+});
+
+const invoiceSchema = z.object({
+  companyId: z.string().min(1),
+  customerId: z.string().min(1),
+  issueDate: z.string(),
+  deliveryDate: z.string().optional(),
+  dueDate: z.string(),
+  notes: z.string().optional(),
+  items: z.array(itemSchema).min(1),
+  netTotal: z.number(),
+  taxTotal: z.number(),
+  grossTotal: z.number(),
+});
+
+export async function POST(req: NextRequest) {
+  const session = await auth();
+  if (!session?.user?.id) return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+
+  const body = await req.json();
+  const parsed = invoiceSchema.safeParse(body);
+  if (!parsed.success) return NextResponse.json({ error: parsed.error.issues }, { status: 400 });
+
+  const { items, companyId, ...invoiceData } = parsed.data;
+
+  // Verify company belongs to user
+  const company = await prisma.company.findFirst({
+    where: { id: companyId, userId: session.user.id },
+  });
+  if (!company) return NextResponse.json({ error: "Company not found" }, { status: 404 });
+
+  const number = await generateInvoiceNumber(companyId);
+
+  const invoice = await prisma.invoice.create({
+    data: {
+      ...invoiceData,
+      number,
+      companyId,
+      issueDate: new Date(invoiceData.issueDate),
+      deliveryDate: invoiceData.deliveryDate ? new Date(invoiceData.deliveryDate) : null,
+      dueDate: new Date(invoiceData.dueDate),
+      items: {
+        create: items.map((item) => ({
+          ...item,
+          quantity: item.quantity,
+          unitPrice: item.unitPrice,
+          taxRate: item.taxRate,
+          netAmount: item.netAmount,
+          taxAmount: item.taxAmount,
+          grossAmount: item.grossAmount,
+        })),
+      },
+    },
+    include: { items: true, customer: true },
+  });
+
+  return NextResponse.json(invoice, { status: 201 });
+}