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-totals.test.ts

@@ -0,0 +1,94 @@
+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
+  });
+});