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/context/AlertContext.tsx

@@ -5,6 +5,7 @@ import {
   useCallback,
   useMemo,
   useRef,
+  useEffect,
   type ReactNode,
 } from "react";
 
@@ -39,6 +40,15 @@ export function AlertProvider({ children }: { children: ReactNode }) {
   }, []);
 
   const counterRef = useRef(0);
+  const timeoutsRef = useRef<Set<ReturnType<typeof setTimeout>>>(new Set());
+
+  useEffect(() => {
+    return () => {
+      timeoutsRef.current.forEach(clearTimeout);
+      timeoutsRef.current.clear();
+    };
+  }, []);
+
   const addAlert = useCallback(
     (message: string, type = "success", duration = 4000) => {
       const id = `${Date.now()}-${counterRef.current++}`;
@@ -47,7 +57,8 @@ export function AlertProvider({ children }: { children: ReactNode }) {
         { id, message, type: type as Alert["type"] },
       ]);
       if (duration > 0) {
-        setTimeout(() => removeAlert(id), duration);
+        const timeoutId = setTimeout(() => removeAlert(id), duration);
+        timeoutsRef.current.add(timeoutId);
       }
       return id;
     },