Add schema.org structured data, Vorkasse email notice, newsletter duplicate detection

- Organization (site-wide), Product (/todo-cards), BlogPosting (every
  /blog/[slug]) JSON-LD via new app/lib/structuredData.ts — no new
  Payload fields needed, derived from existing data. Verified locally
  by curling each page and checking the rendered script tag.
- Order confirmation email gains the same "please transfer to this
  account, processed after payment received" notice the invoice PDF
  already had for Vorkasse orders — OrderConfirmationData's new
  isManualPayment flag is set explicitly by each caller (never derived
  from paymentMethodTitle, which already broke once this session after
  a payment-methods rename). CompanySettings gains bankName (existed on
  the backend, was missing from the frontend's type/usage).
- Newsletter signup now detects an already-subscribed email
  (verified empirically: Brevo's doubleOptinConfirmation endpoint gives
  identical 201 responses for new vs. already-confirmed contacts) via a
  GET /v3/contacts/{email} pre-check, and shows a distinct message
  instead of silently resending the confirmation mail. Success message
  text centralized in useNewsletterSignup.ts instead of duplicated
  across 4 forms.
- Bumped @einfach-produktiv/invoicing to the version with the
  unpaid-notice layout fix (full width, more top spacing — was
  squeezed into the narrow paid-badge column).

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

app/api/checkout/route.ts

@@ -444,6 +444,7 @@ export async function POST(request: Request) {
         discountAmount,
         discountCode: body.discountCode || null,
         total,
+        isManualPayment: true,
       },
       body.email,
     ).catch((err) => {

app/api/newsletter/subscribe/route.ts

@@ -26,5 +26,5 @@ export async function POST(req: Request) {
   if (!result.ok) {
     return NextResponse.json({ ok: false, reason: "Anmeldung ist fehlgeschlagen. Bitte versuche es später erneut." }, { status: 502 });
   }
-  return NextResponse.json({ ok: true });
+  return NextResponse.json({ ok: true, alreadySubscribed: result.alreadySubscribed ?? false });
 }