Fix cache headers: force revalidation on content, correct _astro path

- Add Cache-Control: no-cache as default so browsers revalidate via ETag
  instead of heuristic caching stale llms.txt and HTML pages
- Fix hashed asset path from /docs/_astro/* to /_astro/* (root is /srv)
- _astro/* immutable rule still applies for hashed Vite bundles

docs/Dockerfile

@@ -30,9 +30,10 @@ COPY <<'CADDYFILE' /etc/caddy/Caddyfile
 		X-Content-Type-Options nosniff
 		X-Frame-Options DENY
 		Referrer-Policy strict-origin-when-cross-origin
+		Cache-Control "no-cache"
 	}
 
-	header /docs/_astro/* {
+	header /_astro/* {
 		Cache-Control "public, max-age=31536000, immutable"
 	}
 }