feat: SEO improvements — Twitter cards, canonical URL, sitemap, robots, llms.txt

Adds metadataBase, og:image, og:url, Twitter card tags, canonical
URL, sitemap.ts, robots.ts with AI crawler directives, and llms.txt.

apps/web/src/app/sitemap.ts

@@ -0,0 +1,8 @@
+import { MetadataRoute } from "next";
+
+export default function sitemap(): MetadataRoute.Sitemap {
+  const baseUrl = "https://codeboard.vectry.tech";
+  return [
+    { url: baseUrl, lastModified: new Date(), changeFrequency: "weekly", priority: 1.0 },
+  ];
+}