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/app/core/sitemap.spec.ts

@@ -0,0 +1,49 @@
+import { describe, expect, it } from 'vitest';
+
+import { buildSitemap } from './sitemap';
+
+const xml = buildSitemap('https://pireon.dev', {
+  pages: [
+    {
+      id: 1,
+      updated_at: '2026-09-27T20:36:14+00:00',
+      alternates: {
+        es: '/sobre-nosotros',
+        en: '/en/about-us',
+        ca: '/ca/qui-som',
+      },
+    },
+  ],
+});
+
+describe('buildSitemap', () => {
+  it('lists every language of a record as its own url', () => {
+    for (const path of ['/sobre-nosotros', '/en/about-us', '/ca/qui-som']) {
+      expect(xml).toContain(`<loc>https://pireon.dev${path}</loc>`);
+    }
+  });
+
+  it('states every language on each url, with the default as x-default', () => {
+    expect(xml).toContain(
+      'hreflang="en" href="https://pireon.dev/en/about-us"',
+    );
+    expect(xml).toContain(
+      'hreflang="x-default" href="https://pireon.dev/sobre-nosotros"',
+    );
+    expect(xml).toContain('<lastmod>2026-09-27T20:36:14+00:00</lastmod>');
+  });
+
+  it('includes the landing and the listings this site composes', () => {
+    expect(xml).toContain('<loc>https://pireon.dev/</loc>');
+    // `&` escaped, as XML requires.
+    expect(xml).toContain('<loc>https://pireon.dev/?lang=en</loc>');
+    expect(xml).toContain('<loc>https://pireon.dev/radar</loc>');
+    expect(xml).toContain('<loc>https://pireon.dev/ca/col-labora</loc>');
+  });
+
+  it('is a well-formed document', () => {
+    const document = new DOMParser().parseFromString(xml, 'application/xml');
+
+    expect(document.getElementsByTagName('parsererror')).toHaveLength(0);
+  });
+});