align nginx and Caddy config with official Nextcloud docs

Move security headers to Caddy (edge proxy), remove nginx gzip
(Caddy already compresses), add asset_immutable map for versioned
cache control, add missing static file extensions, fix .well-known
block, and hide X-Powered-By header.

caddy/Caddyfile

@@ -14,6 +14,11 @@ nextcloud.t-gstone.de {
 	reverse_proxy nextcloud-nginx:80
 
 	header Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
+	header Referrer-Policy "no-referrer"
+	header X-Content-Type-Options "nosniff"
+	header X-Frame-Options "SAMEORIGIN"
+	header X-Permitted-Cross-Domain-Policies "none"
+	header X-Robots-Tag "noindex, nofollow"
 
 	request_body {
 		max_size 10G