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/services/invoices.service.ts

@@ -1,5 +1,6 @@
 import prisma from "../config/database";
 import { utcMidnightOfLocalDay } from "../utils/date";
+import { lineNet } from "../utils/money";
 import { toCzk } from "./exchange-rates";
 import {
   generateInvoiceNumber,
@@ -31,6 +32,8 @@ interface InvoiceItemInput {
   quantity?: number;
   unit?: string | null;
   unit_price?: number;
+  /** Per-item discount as a percentage (0–100). */
+  discount?: number;
   vat_rate?: number;
   position?: number;
 }
@@ -165,14 +168,24 @@ function buildInvoiceWhere(
 }
 
 function computeInvoiceTotals(
-  items: Array<{ quantity: unknown; unit_price: unknown; vat_rate: unknown }>,
+  items: Array<{
+    quantity: unknown;
+    unit_price: unknown;
+    discount?: unknown;
+    vat_rate: unknown;
+  }>,
   applyVat: boolean | null,
   defaultVatRate: unknown,
 ) {
   const { subtotal, vat: vatAmount } = computeMoney(
     items,
     applyVat,
-    (i) => (Number(i.quantity) || 0) * (Number(i.unit_price) || 0),
+    (i) =>
+      lineNet(
+        Number(i.quantity) || 0,
+        Number(i.unit_price) || 0,
+        Number(i.discount),
+      ),
     // Preserve original semantics: a line vat_rate of 0 (non-null, non-"") IS used.
     (i) =>
       i.vat_rate != null && i.vat_rate !== ""
@@ -305,6 +318,7 @@ export function invoiceTotalWithVat(inv: {
   invoice_items: Array<{
     quantity: { toNumber(): number } | null;
     unit_price: { toNumber(): number } | null;
+    discount?: { toNumber(): number } | null;
     vat_rate: { toNumber(): number } | null;
   }>;
 }) {
@@ -312,8 +326,11 @@ export function invoiceTotalWithVat(inv: {
     inv.invoice_items,
     inv.apply_vat,
     (i) =>
-      (Number(i.quantity?.toNumber()) || 0) *
-      (Number(i.unit_price?.toNumber()) || 0),
+      lineNet(
+        Number(i.quantity?.toNumber()) || 0,
+        Number(i.unit_price?.toNumber()) || 0,
+        i.discount?.toNumber(),
+      ),
     // Preserve original `||` semantics: a line rate of 0/NaN falls through to
     // the invoice default, then 21.
     (i) =>
@@ -401,7 +418,12 @@ export async function getInvoiceStats(queryMonth?: number, queryYear?: number) {
     const { vat } = computeMoney(
       inv.invoice_items,
       inv.apply_vat,
-      (item) => (Number(item.quantity) || 0) * (Number(item.unit_price) || 0),
+      (item) =>
+        lineNet(
+          Number(item.quantity) || 0,
+          Number(item.unit_price) || 0,
+          Number(item.discount),
+        ),
       (item) => Number(item.vat_rate) || Number(inv.vat_rate) || 21,
       false, // accumulate raw per-line VAT; round only the per-currency sum
     );
@@ -551,6 +573,7 @@ export async function createInvoice(body: InvoiceInput) {
           quantity: item.quantity ?? 1,
           unit: item.unit ?? null,
           unit_price: item.unit_price ?? 0,
+          discount: item.discount ?? 0,
           vat_rate: item.vat_rate ?? 21.0,
           position: item.position ?? i,
         })),
@@ -688,6 +711,7 @@ export async function updateInvoice(id: number, body: InvoiceInput) {
             quantity: item.quantity ?? 1,
             unit: item.unit ?? null,
             unit_price: item.unit_price ?? 0,
+            discount: item.discount ?? 0,
             vat_rate: item.vat_rate ?? 21.0,
             position: item.position ?? i,
           })),