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/sitemap.xml.get.ts

@@ -0,0 +1,34 @@
+/**
+ * `/sitemap.xml`, from the API's `/slugs` — one request for every published
+ * address, instead of paging through each collection.
+ *
+ * Cached like any page: a record published a minute ago can wait a minute to
+ * be listed. If the API is down the answer is a 503 rather than a sitemap
+ * with half the site missing from it, which a crawler would believe.
+ */
+import {
+  defineEventHandler,
+  getRequestHeaders,
+  setResponseHeader,
+  setResponseStatus,
+} from 'h3';
+
+import { pireonApi } from '../../app/core/api';
+import { originFromHeaders } from '../../app/core/origin';
+import { buildSitemap } from '../../app/core/sitemap';
+
+export default defineEventHandler(async (event) => {
+  const origin = originFromHeaders(getRequestHeaders(event)) ?? '';
+  const { data } = await pireonApi.slugs();
+
+  if (!data) {
+    setResponseStatus(event, 503);
+    setResponseHeader(event, 'Cache-Control', 'no-store');
+
+    return 'Sitemap temporarily unavailable';
+  }
+
+  setResponseHeader(event, 'Content-Type', 'application/xml; charset=utf-8');
+
+  return buildSitemap(origin, data);
+});