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/blog/[slug]/page.tsx

@@ -7,8 +7,9 @@ import { Reveal } from "../../components/Reveal";
 import { Footer } from "../../components/Footer";
 import { RichText } from "../../components/RichText";
 import { LivePostContent } from "./components/LivePostContent";
-import { getBlogPosts, getPostBySlug } from "../../lib/payload";
+import { getBlogPosts, getPostBySlug, getCompanySettings } from "../../lib/payload";
 import { formatDate } from "../../lib/format";
+import { buildArticleSchema } from "../../lib/structuredData";
 
 export async function generateMetadata({
   params,
@@ -61,9 +62,12 @@ export default async function BlogDetailPage({
   // showing a fake/duplicate card when this is the only post.
   const otherPosts = await getBlogPosts(4);
   const nextPost = otherPosts.find((p) => p.slug !== post.slug) ?? null;
+  const seller = await getCompanySettings();
+  const articleSchema = buildArticleSchema(post, seller);
 
   return (
     <>
+      <script type="application/ld+json" dangerouslySetInnerHTML={{ __html: JSON.stringify(articleSchema) }} />
       <main className="flex flex-col flex-1 bg-bg-base">
         {isPreview ? (
           <LivePostContent initialPost={post} />