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/config/env.ts

@@ -56,7 +56,14 @@ export const config = {
     path: process.env.NAS_PATH || "Z:/02_PROJEKTY",
     financialsPath: process.env.NAS_FINANCIALS_PATH || "",
     offersPath: process.env.NAS_OFFERS_PATH || "",
-    maxUploadSize: parseInt(process.env.MAX_UPLOAD_SIZE || "52428800", 10),
+    maxUploadSize: (() => {
+      const parsed = parseInt(process.env.MAX_UPLOAD_SIZE || "52428800", 10);
+      if (Number.isNaN(parsed) || parsed <= 0) {
+        console.warn("Invalid MAX_UPLOAD_SIZE, using default 52428800");
+        return 52428800;
+      }
+      return parsed;
+    })(),
   },
 
   email: {