Sync newsletter opt-ins to Brevo

Both standalone signup forms (Newsletter.tsx on Home/newsletter page,
NewsletterModal.tsx from the Navbar CTA) were previously non-functional
— static markup with no onSubmit/state at all, nothing was ever
captured. They're now real client forms posting to the new
/api/newsletter/subscribe route, which upserts the contact into
Brevo's Contacts API (list id from BREVO_LIST_ID). Checkout's existing
newsletterOptIn checkbox gets the same sync, fire-and-forget alongside
the order-confirmation email — a failed marketing sync must never
fail checkout.

lib/brevo.ts is the only thing that talks to Brevo; this app still
never sends marketing mail itself. Whatever automation Brevo has
configured on the list (Welcome Flow etc.) runs entirely on their
side — Brevo's Automation workflows aren't manageable via their
public API at all, so that part can't be wired up from here.

Needs BREVO_API_KEY and BREVO_LIST_ID set in the frontend's Coolify
environment — not yet added there.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/api/checkout/route.ts

@@ -11,6 +11,7 @@ import { sendOrderConfirmationEmail } from "../../lib/orderEmail";
 import { normalizeVatId, isValidVatId } from "../../lib/vatId";
 import { checkVatIdViaVies } from "../../lib/vies";
 import { computeExemptTotals, destinationCountry, isExemptionEligibleCountry } from "../../lib/vatExemption";
+import { upsertNewsletterContact } from "../../lib/brevo";
 
 // Plain float arithmetic on money (quantity × unitPrice summed across
 // lines, a percent discount, subtracting/adding those together) drifts
@@ -376,6 +377,13 @@ export async function POST(request: Request) {
     });
   });
 
+  // Fire-and-forget, same reasoning as the confirmation email above — a
+  // failed marketing sync is not worth failing checkout over, and doesn't
+  // even need a critical alert (nothing customer-facing depends on it).
+  if (body.newsletterOptIn) {
+    upsertNewsletterContact(body.email, "checkout").catch(() => {});
+  }
+
   return NextResponse.json({
     ok: true,
     orderNumber: order.orderNumber,