security: fix all Critical and High findings from FLAWS_REPORT audit

- Auth: pessimistic locking on login tokens and refresh token rotation,
  backup code attempt counter, rate limiting verification
- Schema: unique constraints on business numbers, FK relations,
  unsigned/signed alignment, attendance duplicate prevention
- Invoices/PDFs: DOMPurify sanitization, bounded queries in stats
  and alerts, VAT rounding, Puppeteer error handling
- Orders/Offers: transactional parent+child creation, Zod NaN
  refinement, status enums, uniqueness checks
- Projects/Files: path traversal protection, streamed uploads,
  permission guards, query param validation
- Attendance/HR: duplicate checks, ownership validation, GPS
  restrictions, trip distance validation
- Frontend: modal lock reference counting, XSS escaping in print
  HTML, ref mutation fixes, accessibility attributes

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/admin/pages/OfferDetail.tsx

@@ -55,9 +55,6 @@ interface OfferItem {
   is_included_in_total: boolean;
 }
 
-let _itemKeyCounter = 0;
-const nextItemKey = () => `item-${++_itemKeyCounter}`;
-
 interface ScopeSection {
   title: string;
   title_cz: string;
@@ -113,16 +110,6 @@ const emptyScopeSection = (): ScopeSection => ({
   content: "",
 });
 
-const emptyItem = (): OfferItem => ({
-  _key: nextItemKey(),
-  description: "",
-  item_description: "",
-  quantity: 1,
-  unit: "ks",
-  unit_price: 0,
-  is_included_in_total: true,
-});
-
 function SortableItemRow({
   item,
   index,
@@ -288,11 +275,25 @@ export default function OfferDetail() {
     useSensor(KeyboardSensor),
   );
 
+  const itemKeyCounter = useRef(0);
+  const emptyItem = useCallback(
+    (): OfferItem => ({
+      _key: `item-${++itemKeyCounter.current}`,
+      description: "",
+      item_description: "",
+      quantity: 1,
+      unit: "ks",
+      unit_price: 0,
+      is_included_in_total: true,
+    }),
+    [],
+  );
+
   const [loading, setLoading] = useState(isEdit);
   const [saving, setSaving] = useState(false);
   const [errors, setErrors] = useState<Record<string, string | undefined>>({});
   const [form, setForm] = useState<OfferForm>(emptyForm);
-  const [items, setItems] = useState<OfferItem[]>([emptyItem()]);
+  const [items, setItems] = useState<OfferItem[]>(() => [emptyItem()]);
   const [sections, setSections] = useState<ScopeSection[]>([]);
   const [scopeTemplates, setScopeTemplates] = useState<
     Array<{
@@ -397,7 +398,10 @@ export default function OfferDetail() {
         });
         setItems(
           d.items?.length
-            ? d.items.map((it: any) => ({ ...it, _key: nextItemKey() }))
+            ? d.items.map((it: any) => ({
+                ...it,
+                _key: `item-${++itemKeyCounter.current}`,
+              }))
             : [emptyItem()],
         );
         setSections(