Tell search engines and link previews what each page is

- /sitemap.xml, from the API's /slugs plus the landing and listings:
  every address in every language, with hreflang and x-default. A 503
  when the API is down, rather than a sitemap missing half the site.
- /robots.txt, naming the sitemap on whatever domain this is served on.
- Slugs renamed in the panel now redirect. The API has always kept the
  old address the moment one changed; middleware/redirects.ts answers
  it with a 301, from a list kept for a minute and through outages.
- og:url, og:image (the record's own picture, the logo otherwise) and
  schema.org JSON-LD: Organization on the landing; WebPage,
  CreativeWork, Person or Service elsewhere. Shared through core/head.
- The proxy-derived origin moves to core/origin.ts, used by the page
  shell, the landing and both server routes.

In development only /api/* reaches Nitro, so the two files 404 under
`npm start`; they were checked against a production build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

landing/src/server/routes/robots.txt.get.ts

@@ -0,0 +1,21 @@
+/**
+ * Everything is crawlable, and the sitemap says where everything is. Absolute,
+ * because robots.txt requires it, from whichever domain this is served on.
+ */
+import { defineEventHandler, getRequestHeaders, setResponseHeader } from 'h3';
+
+import { originFromHeaders } from '../../app/core/origin';
+
+export default defineEventHandler((event) => {
+  const origin = originFromHeaders(getRequestHeaders(event)) ?? '';
+
+  setResponseHeader(event, 'Content-Type', 'text/plain; charset=utf-8');
+
+  return [
+    'User-agent: *',
+    'Allow: /',
+    '',
+    `Sitemap: ${origin}/sitemap.xml`,
+    '',
+  ].join('\n');
+});