docs-site: serve the branded 404 page with a real 404 status

Missing paths returned 404 with an EMPTY body — the built 404.html was
never reached. handle_errors routes 404s to it, status stays honest.

docs-site/Caddyfile

@@ -8,4 +8,14 @@
 
     @hashed path /_astro/*
     header @hashed Cache-Control "public, max-age=31536000, immutable"
+
+    # Serve the branded 404 page with a real 404 status. It was already in
+    # the build, just never reached — Caddy returned an empty error body.
+    handle_errors {
+        @404 expression {err.status_code} == 404
+        handle @404 {
+            rewrite * /404.html
+            file_server
+        }
+    }
 }