fix(seo): add HSTS headers, clean crawler directives, and exclude /links from sitemap

public/_headers

@@ -0,0 +1,5 @@
+/*
+  Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
+  X-Content-Type-Options: nosniff
+  X-Frame-Options: SAMEORIGIN
+  Referrer-Policy: strict-origin-when-cross-origin

public/robots.txt

@@ -14,20 +14,5 @@ User-agent: Googlebot-Image
 Disallow: /gallery
 Disallow: /gallery/
 
-# Allow specific crawlers
-User-agent: Googlebot
-Allow: /
-Disallow: /api/
-Disallow: /.vercel/
-Disallow: /.astro/
-Disallow: /maintenance
-Disallow: /gallery
-Disallow: /gallery/
-
-User-agent: Bingbot
-Allow: /
-Disallow: /gallery
-Disallow: /gallery/
-
 # Reference to sitemap
 Sitemap: https://crischarles.com/sitemap-index.xml