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/admin/components/document/DocumentItemsEditor.tsx

@@ -49,10 +49,31 @@ export interface DocumentItem {
   quantity: string | number;
   unit: string;
   unit_price: string | number;
+  /** Per-item discount as a percentage (0–100). Offers only; 0 elsewhere. */
+  discount: string | number;
   /** Offers only — undefined (issued orders) counts as included. */
   is_included_in_total?: boolean;
 }
 
+/**
+ * Discount input styling: centered, and with the native number spinners hidden
+ * so a 3-digit value ("100") isn't clipped under the up/down arrows.
+ */
+const discountInputSx = {
+  "& input": { textAlign: "center" },
+  "& input[type=number]": { MozAppearance: "textfield" as const },
+  "& input::-webkit-outer-spin-button, & input::-webkit-inner-spin-button": {
+    WebkitAppearance: "none",
+    margin: 0,
+  },
+};
+
+/** NET of one line after its per-item percentage discount. */
+const discountedLineNet = (item: DocumentItem): number =>
+  (Number(item.quantity) || 0) *
+  (Number(item.unit_price) || 0) *
+  (1 - (Number(item.discount) || 0) / 100);
+
 let documentItemKeyCounter = 0;
 
 /** Unique client-side key for a (new or hydrated) document item row. */
@@ -66,6 +87,7 @@ export const emptyDocumentItem = (): DocumentItem => ({
   quantity: 1,
   unit: "ks",
   unit_price: 0,
+  discount: 0,
   is_included_in_total: true,
 });
 
@@ -73,9 +95,7 @@ export const emptyDocumentItem = (): DocumentItem => ({
 export const documentItemsTotal = (items: DocumentItem[]): number =>
   items.reduce(
     (sum, it) =>
-      it.is_included_in_total === false
-        ? sum
-        : sum + (Number(it.quantity) || 0) * (Number(it.unit_price) || 0),
+      it.is_included_in_total === false ? sum : sum + discountedLineNet(it),
     0,
   );
 
@@ -111,6 +131,7 @@ function SortableItemRow({
   readOnly,
   canDelete,
   showIncludedInTotal,
+  showDiscount,
   itemDescriptionMaxLength,
   onUpdate,
   onRemove,
@@ -121,6 +142,7 @@ function SortableItemRow({
   readOnly: boolean;
   canDelete: boolean;
   showIncludedInTotal: boolean;
+  showDiscount: boolean;
   itemDescriptionMaxLength: number;
   onUpdate: (field: keyof DocumentItem, value: string | boolean) => void;
   onRemove: () => void;
@@ -143,8 +165,7 @@ function SortableItemRow({
     position: "relative" as const,
     zIndex: isDragging ? 10 : undefined,
   };
-  const lineTotal =
-    (Number(item.quantity) || 0) * (Number(item.unit_price) || 0);
+  const lineTotal = discountedLineNet(item);
 
   if (isMobile) {
     return (
@@ -253,6 +274,17 @@ function SortableItemRow({
             InputProps={{ readOnly }}
             sx={{ "& input": { textAlign: "right" } }}
           />
+          {showDiscount && (
+            <TextField
+              label="Sleva %"
+              type="number"
+              value={item.discount ?? ""}
+              onChange={(e) => onUpdate("discount", e.target.value)}
+              slotProps={{ htmlInput: { min: "0", max: "100", step: "any" } }}
+              InputProps={{ readOnly }}
+              sx={discountInputSx}
+            />
+          )}
         </Box>
         <Box
           sx={{
@@ -396,6 +428,18 @@ function SortableItemRow({
           sx={{ "& input": { textAlign: "right" } }}
         />
       </TableCell>
+      {showDiscount && (
+        <TableCell>
+          <TextField
+            type="number"
+            value={item.discount ?? ""}
+            onChange={(e) => onUpdate("discount", e.target.value)}
+            slotProps={{ htmlInput: { min: "0", max: "100", step: "any" } }}
+            InputProps={{ readOnly }}
+            sx={discountInputSx}
+          />
+        </TableCell>
+      )}
       {showIncludedInTotal && (
         <TableCell align="center">
           <Checkbox
@@ -442,6 +486,8 @@ interface DocumentItemsEditorProps {
   error?: string;
   /** Render the offers-only "V ceně" (is_included_in_total) column. */
   showIncludedInTotal?: boolean;
+  /** Render the per-item "Sleva %" discount column (offers only). */
+  showDiscount?: boolean;
   /** Schema cap for the detailed description (offers 8000, issued orders 5000). */
   itemDescriptionMaxLength?: number;
   /** Optional page-specific control (e.g. item-template Select) next to the add button. */
@@ -460,6 +506,7 @@ export default function DocumentItemsEditor({
   readOnly,
   error,
   showIncludedInTotal = false,
+  showDiscount = false,
   itemDescriptionMaxLength = 5000,
   templatesSlot,
 }: DocumentItemsEditorProps) {
@@ -563,6 +610,7 @@ export default function DocumentItemsEditor({
                   readOnly={readOnly}
                   canDelete={items.length > 1}
                   showIncludedInTotal={showIncludedInTotal}
+                  showDiscount={showDiscount}
                   itemDescriptionMaxLength={itemDescriptionMaxLength}
                   onUpdate={(field, value) => updateItem(index, field, value)}
                   onRemove={() => removeItem(index)}
@@ -594,6 +642,11 @@ export default function DocumentItemsEditor({
                     <TableCell sx={{ width: "8rem" }} align="center">
                       Jedn. cena
                     </TableCell>
+                    {showDiscount && (
+                      <TableCell sx={{ width: "7rem" }} align="center">
+                        Sleva %
+                      </TableCell>
+                    )}
                     {showIncludedInTotal && (
                       <TableCell sx={{ width: "4rem" }} align="center">
                         V ceně
@@ -615,6 +668,7 @@ export default function DocumentItemsEditor({
                       readOnly={readOnly}
                       canDelete={items.length > 1}
                       showIncludedInTotal={showIncludedInTotal}
+                      showDiscount={showDiscount}
                       itemDescriptionMaxLength={itemDescriptionMaxLength}
                       onUpdate={(field, value) =>
                         updateItem(index, field, value)