Add backend unit/integration tests, Playwright e2e tests, README, cookie Secure fix

backend/src/utils.ts

@@ -0,0 +1,19 @@
+export function toCents(price: string | number): number {
+  const n = typeof price === 'string' ? parseFloat(price) : price;
+  if (Number.isNaN(n) || n < 0) {
+    throw new Error('invalid price');
+  }
+  return Math.round(n * 100);
+}
+
+export function formatPrice(cents: number): string {
+  return `$${(cents / 100).toFixed(2)}`;
+}
+
+const EMAIL_RE = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
+export function isValidEmail(email: string): boolean {
+  return EMAIL_RE.test(email.trim());
+}
+
+export const MARKETING_CONSENT_TEXT =
+  'I want to receive occasional emails about new one-of-a-kind items from Redefined Designs. I can unsubscribe at any time.';