feat(documents): per-item Sleva (% discount) on offers + invoices

Adds an optional per-line-item percentage discount ("Sleva") to offers and
issued invoices, on both the detail-page editor and the PDF.

- DB: discount Decimal(5,2) DEFAULT 0 on quotation_items + invoice_items
  (migration 20260613195833_add_item_discount).
- Totals/VAT: new shared lineNet() (qty × price × (1 − discount/100)); offers'
  NET total and invoices' subtotal AND VAT now compute on the discounted net
  (every getBase/enrichQuotation/stats path). Backward compatible: discount 0
  is identical to before.
- Schemas: discount = numberInRange(0,100), default 0; persisted on
  create/update (+ offer duplicate).
- Detail editors: editable "Sleva %" column — offers via DocumentItemsEditor's
  new opt-in showDiscount prop (issued orders unaffected); invoices in their own
  editor. Spinners hidden + 7rem column so "100" is fully visible. Read-only
  invoice view shows the column when any line has a discount.
- PDFs (offers + invoices): conditional "Sleva"/"Discount" column rendered only
  when an item has a discount; line/total/VAT always use the discounted net.

Tests: +16 (money, totals incl. VAT-on-discounted-net, schema range, PDF
column present/absent). Full suite 665 pass, tsc -b clean, lint 0. Editor
verified live via Playwright. Design spec in docs/superpowers/specs/.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/__tests__/item-discount-pdf.test.ts

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+import { describe, it, expect, beforeAll, afterAll } from "vitest";
+import Fastify from "fastify";
+import jwt from "jsonwebtoken";
+import prisma from "../config/database";
+import { config } from "../config/env";
+import offersPdfRoutes from "../routes/admin/offers-pdf";
+import invoicesPdfRoutes from "../routes/admin/invoices-pdf";
+import { createInvoice } from "../services/invoices.service";
+
+/**
+ * The per-item Sleva column is CONDITIONAL on the PDF: render it only when at
+ * least one item carries a discount > 0; otherwise the table looks exactly as
+ * before. The discounted net always flows into the line/grand totals.
+ */
+
+let app: ReturnType<typeof Fastify> | null = null;
+let adminToken = "";
+const quotationIds: number[] = [];
+const invoiceIds: number[] = [];
+
+beforeAll(async () => {
+  app = Fastify({ logger: false });
+  await app.register(offersPdfRoutes, { prefix: "/api/admin/offers-pdf" });
+  await app.register(invoicesPdfRoutes, { prefix: "/api/admin/invoices-pdf" });
+  const admin = await prisma.users.findFirst({
+    where: { roles: { name: "admin" } },
+  });
+  if (!admin) throw new Error("Test setup: admin user not found");
+  adminToken = jwt.sign(
+    { sub: admin.id, username: admin.username, role: "admin" },
+    config.jwt.secret,
+    { expiresIn: "15m" },
+  );
+});
+
+afterAll(async () => {
+  for (const id of quotationIds)
+    await prisma.quotations.deleteMany({ where: { id } });
+  for (const id of invoiceIds)
+    await prisma.invoices.deleteMany({ where: { id } });
+  if (app) await app.close();
+});
+
+const offerHtml = async (discount: number): Promise<string> => {
+  const q = await prisma.quotations.create({
+    data: {
+      quotation_number: `Q-SLEVA-${discount}-${Date.now()}`,
+      status: "active",
+      currency: "CZK",
+      language: "cs",
+      quotation_items: {
+        create: [
+          {
+            description: "Položka",
+            quantity: 2,
+            unit: "ks",
+            unit_price: 100,
+            discount,
+            is_included_in_total: true,
+            position: 0,
+          },
+        ],
+      },
+    },
+  });
+  quotationIds.push(q.id);
+  const res = await app!.inject({
+    method: "GET",
+    url: `/api/admin/offers-pdf/${q.id}`,
+    headers: { Authorization: `Bearer ${adminToken}` },
+  });
+  expect(res.statusCode).toBe(200);
+  return res.body;
+};
+
+describe("offer PDF Sleva column", () => {
+  it("renders a Sleva column + discounted total when an item has a discount", async () => {
+    const html = await offerHtml(10);
+    expect(html).toContain("Sleva");
+    // 2 × 100 with 10% off => 180,00 (no thousands separator at this size).
+    expect(html).toContain("180,00");
+    expect(html).not.toContain("200,00");
+  });
+
+  it("omits the Sleva column when no item has a discount", async () => {
+    const html = await offerHtml(0);
+    expect(html).not.toContain("Sleva");
+    expect(html).toContain("200,00");
+  });
+});
+
+const invoiceHtml = async (discount: number): Promise<string> => {
+  const inv = await createInvoice({
+    status: "draft",
+    currency: "CZK",
+    apply_vat: true,
+    vat_rate: 21,
+    language: "cs",
+    items: [
+      {
+        description: "Položka",
+        quantity: 2,
+        unit_price: 1000,
+        discount,
+        vat_rate: 21,
+      },
+    ],
+  });
+  invoiceIds.push(inv.id);
+  const res = await app!.inject({
+    method: "GET",
+    url: `/api/admin/invoices-pdf/${inv.id}`,
+    headers: { Authorization: `Bearer ${adminToken}` },
+  });
+  expect(res.statusCode).toBe(200);
+  return res.body;
+};
+
+describe("invoice PDF Sleva column", () => {
+  it("renders a Sleva column when an item has a discount", async () => {
+    const html = await invoiceHtml(10);
+    expect(html).toContain("Sleva");
+    expect(html).toContain("10");
+  });
+
+  it("omits the Sleva column when no item has a discount", async () => {
+    const html = await invoiceHtml(0);
+    expect(html).not.toContain("Sleva");
+  });
+});

src/__tests__/item-discount-schema.test.ts

@@ -0,0 +1,55 @@
+import { describe, it, expect } from "vitest";
+import { CreateQuotationSchema } from "../schemas/offers.schema";
+import { CreateInvoiceSchema } from "../schemas/invoices.schema";
+
+/**
+ * The per-item Sleva is a percentage: the offer- and invoice-item schemas must
+ * accept 0–100, default to 0 when absent, and reject out-of-range values.
+ */
+
+describe("offer item discount schema", () => {
+  it("accepts a 0–100 discount", () => {
+    const r = CreateQuotationSchema.safeParse({
+      items: [{ description: "X", quantity: 1, unit_price: 100, discount: 15 }],
+    });
+    expect(r.success).toBe(true);
+    expect(r.success && r.data.items?.[0].discount).toBe(15);
+  });
+
+  it("defaults discount to 0 when absent", () => {
+    const r = CreateQuotationSchema.safeParse({
+      items: [{ description: "X", quantity: 1, unit_price: 100 }],
+    });
+    expect(r.success && r.data.items?.[0].discount).toBe(0);
+  });
+
+  it("rejects a discount above 100", () => {
+    const r = CreateQuotationSchema.safeParse({
+      items: [
+        { description: "X", quantity: 1, unit_price: 100, discount: 150 },
+      ],
+    });
+    expect(r.success).toBe(false);
+  });
+});
+
+describe("invoice item discount schema", () => {
+  it("accepts a 0–100 discount and defaults to 0", () => {
+    const ok = CreateInvoiceSchema.safeParse({
+      items: [{ description: "X", quantity: 1, unit_price: 100, discount: 10 }],
+    });
+    expect(ok.success && ok.data.items?.[0].discount).toBe(10);
+
+    const def = CreateInvoiceSchema.safeParse({
+      items: [{ description: "X", quantity: 1, unit_price: 100 }],
+    });
+    expect(def.success && def.data.items?.[0].discount).toBe(0);
+  });
+
+  it("rejects a negative discount", () => {
+    const r = CreateInvoiceSchema.safeParse({
+      items: [{ description: "X", quantity: 1, unit_price: 100, discount: -5 }],
+    });
+    expect(r.success).toBe(false);
+  });
+});

src/__tests__/item-discount-totals.test.ts

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+import { describe, it, expect, afterEach } from "vitest";
+import prisma from "../config/database";
+import { createOffer, getOfferTotals } from "../services/offers.service";
+import {
+  createInvoice,
+  getInvoiceListTotals,
+} from "../services/invoices.service";
+
+/**
+ * Per-item Sleva (% discount) must reduce the line NET — and on invoices the
+ * VAT is computed on the discounted net. Hits the real `app_test` DB through the
+ * service layer (suite convention), mirroring offer-invoice-totals.test.ts.
+ */
+
+const MARKER = "SLEVA-TOTALS-2096";
+const YEAR = 2096;
+const MONTH = 8; // invoice issue_date window [2096-08-01, 2096-09-01)
+const dateStr = `${YEAR}-0${MONTH}-15`;
+
+const offerIds: number[] = [];
+const invoiceIds: number[] = [];
+
+afterEach(async () => {
+  for (const id of offerIds)
+    await prisma.quotations.deleteMany({ where: { id } });
+  for (const id of invoiceIds)
+    await prisma.invoices.deleteMany({ where: { id } });
+  offerIds.length = 0;
+  invoiceIds.length = 0;
+  await prisma.number_sequences.deleteMany({
+    where: { type: { in: ["offer", "invoice"] } },
+  });
+});
+
+const amountFor = (
+  totals: { currency: string; amount: number }[],
+  currency: string,
+): number | undefined => totals.find((t) => t.currency === currency)?.amount;
+
+describe("offer per-item discount", () => {
+  it("reduces the offer NET total by the per-item percentage", async () => {
+    // 2 × 1000 with 10% off => 1800 net.
+    const res = await createOffer({
+      status: "draft",
+      currency: "CZK",
+      project_code: MARKER,
+      items: [
+        { description: "X", quantity: 2, unit_price: 1000, discount: 10 },
+      ],
+    });
+    if ("error" in res) throw new Error(JSON.stringify(res));
+    offerIds.push(res.id);
+
+    const { totals } = await getOfferTotals({ search: MARKER });
+    expect(amountFor(totals, "CZK")).toBe(1800);
+  });
+});
+
+describe("invoice per-item discount", () => {
+  it("computes VAT on the discounted net (gross = discounted net + VAT)", async () => {
+    // 2 × 1000 with 10% off => net 1800, VAT@21% = 378, gross 2178.
+    const inv = await createInvoice({
+      status: "draft",
+      currency: "CZK",
+      vat_rate: 21,
+      apply_vat: true,
+      issue_date: dateStr,
+      items: [
+        { description: "X", quantity: 2, unit_price: 1000, discount: 10 },
+      ],
+    });
+    invoiceIds.push(inv.id);
+
+    const { totals } = await getInvoiceListTotals({ month: MONTH, year: YEAR });
+    expect(amountFor(totals, "CZK")).toBe(2178);
+  });
+
+  it("treats a 100% discount line as zero", async () => {
+    const inv = await createInvoice({
+      status: "draft",
+      currency: "CZK",
+      vat_rate: 21,
+      apply_vat: true,
+      issue_date: dateStr,
+      items: [
+        { description: "Free", quantity: 5, unit_price: 200, discount: 100 },
+      ],
+    });
+    invoiceIds.push(inv.id);
+
+    const { totals } = await getInvoiceListTotals({ month: MONTH, year: YEAR });
+    expect(amountFor(totals, "CZK")).toBeUndefined(); // zero totals are dropped
+  });
+});

src/__tests__/money.test.ts

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+import { describe, it, expect } from "vitest";
+import { lineNet } from "../utils/money";
+
+describe("lineNet (per-item discounted net)", () => {
+  it("returns qty × unit_price when there is no discount", () => {
+    expect(lineNet(2, 1000, 0)).toBe(2000);
+  });
+
+  it("applies a percentage discount to the line net", () => {
+    expect(lineNet(2, 1000, 10)).toBe(1800);
+  });
+
+  it("treats a 100% discount as zero", () => {
+    expect(lineNet(3, 500, 100)).toBe(0);
+  });
+
+  it("treats null/undefined/NaN discount as no discount", () => {
+    expect(lineNet(2, 1000, null)).toBe(2000);
+    expect(lineNet(2, 1000, undefined)).toBe(2000);
+    expect(lineNet(2, 1000, NaN)).toBe(2000);
+  });
+});