build: robots meta and robots.txt (noindex, Disallow)

public/index.html

@@ -3,6 +3,7 @@
   <head>
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width,initial-scale=1,viewport-fit=cover" />
+    <meta name="robots" content="noindex, nofollow" />
     <meta name="color-scheme" content="light dark" />
     <meta name="theme-color" content="#F3F4F1" media="(prefers-color-scheme: light)" />
     <meta name="theme-color" content="#141917" media="(prefers-color-scheme: dark)" />

public/robots.txt

@@ -0,0 +1,5 @@
+# bm is a private study app for one group of students: no page is meant
+# for search engines (D-72). nginx also sends X-Robots-Tag: noindex, nofollow
+# on every response, and index.html carries the robots meta.
+User-agent: *
+Disallow: /