docs(spec): per-item Sleva (% discount) on offers + invoices

Approved design: percentage per line item, reduces net (VAT on discounted
net), conditional PDF column, offers + issued invoices only.

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

docs/superpowers/specs/2026-06-13-item-sleva-discount-design.md

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+# Per-item Sleva (% discount) — Offers + Invoices
+
+**Date:** 2026-06-13
+**Status:** Approved → implementing
+**Scope:** Add a per-line-item percentage discount ("Sleva") to **offers**
+(nabídky) and **issued invoices** (Faktury), on both the detail-page item
+editor and the PDF. Received invoices, issued orders and orders are NOT
+touched.
+
+## Decisions (from brainstorming)
+
+- **Type:** percentage per item (0–100), not an absolute amount.
+- **Totals/VAT:** the discount reduces the line **net**; on invoices VAT is
+  computed on the **discounted** net. `net = qty × unit_price × (1 − discount/100)`.
+- **PDF column is conditional:** render the Sleva column only when at least one
+  (included) item has `discount > 0`; otherwise the table is identical to today.
+- **Detail-page column is always visible** (it's the input).
+
+## Data model
+
+Add to `quotation_items` and `invoice_items`:
+
+```
+discount Decimal? @default(0.00) @db.Decimal(5, 2)   // percent, 0–100
+```
+
+Nullable with default `0`, matching the existing money columns. "No sleva" =
+`0` or null. One tracked migration; apply to dev `app` and `app_test`, then
+`prisma generate`.
+
+## Line math (single rule)
+
+`lineNet = qty × unit_price × (1 − clamp(discount,0,100)/100)`
+
+Add a shared backend helper so every site is identical:
+
+```ts
+// src/utils/money.ts (new)
+export const lineNet = (qty: number, unitPrice: number, discountPct: number) =>
+  qty * unitPrice * (1 - (discountPct || 0) / 100);
+```
+
+## Backend
+
+- **Schemas (Zod):** add `discount` to the offer-item and invoice-item schemas
+  via `numberInRange(0, 100)` (shared coercer in `common.ts`), optional, default
+  `0`.
+- **offers.service** (`enrichQuotation`): line total uses `lineNet`; document
+  net ("Celkem bez DPH") sums discounted included lines. Persist `discount` on
+  item create/update.
+- **invoices.service**: every `computeMoney` `getBase` callback
+  (`computeInvoiceTotals`, monthly-stats `invoiceMonthlyAmount`, per-rate
+  `vatMap`) returns `lineNet(...)` so subtotal **and** VAT use the discounted
+  net. Persist `discount` on item create/update.
+- **DocumentItemsEditor's `documentItemsTotal`** (frontend) mirrors the same
+  discounted net.
+
+## Frontend (Section 1 — detail-page editor)
+
+- **Offers** → shared `src/admin/components/document/DocumentItemsEditor.tsx`:
+  `DocumentItem` gains `discount`; add a **"Sleva %"** numeric column (0–100);
+  per-line total and the footer net total apply it. Mobile card layout gets the
+  field. `emptyDocumentItem` defaults `discount: 0`. Hydration + save payloads
+  carry `discount`.
+- **Invoices** → `src/admin/pages/InvoiceDetail.tsx` own editor
+  (`SortableInvoiceRow` + create/edit rows): `InvoiceItem` gains `discount`;
+  same column; net/VAT/gross totals apply it. Mobile layout too.
+
+## PDF (Section 2 — conditional column)
+
+`src/routes/admin/offers-pdf.ts` and `src/routes/admin/invoices-pdf.ts`:
+
+- `const hasDiscount = items.some(i => includedInTotal(i) && Number(i.discount) > 0)`.
+- When `hasDiscount`: add a column — header **"Sleva"** (cs) / **"Discount"**
+  (en, from the document's `language`), cell renders `"<n> %"` (escaped); adjust
+  colspans of any full-width rows accordingly.
+- Line net / document total / VAT always use the discounted net regardless of
+  `hasDiscount`.
+- `escapeHtml` every interpolation (string-built PDF templates).
+
+## Tests
+
+- `offer-invoice-totals` / invoice totals: net, VAT and gross with discounts
+  (incl. a 100% line = 0, and VAT-on-discounted-net).
+- Schema: `discount` rejects <0 and >100, coerces form strings, defaults 0.
+- PDF: column present when an item has a discount; absent when none; values
+  escaped.
+
+## Non-goals (YAGNI)
+
+- No separate "total discount" summary line, no document-level discount.
+- No changes to received invoices, issued orders, or orders.
+
+## Migration etiquette
+
+DB change → user stops the dev server first; create the migration via the
+non-interactive `prisma migrate diff` recipe, `migrate deploy` + `generate` on
+dev and `app_test`, commit schema + migration folder together.