enable Caddy response compression to fix slow page loads

Caddy was decompressing nginx's gzip responses and sending them
uncompressed to the browser, causing core-common.js (5.7MB) to
take 25s to download. Adding encode zstd gzip compresses it to
1.3MB at the edge.

caddy/Caddyfile

@@ -9,6 +9,8 @@
 }
 
 nextcloud.t-gstone.de {
+	encode zstd gzip
+
 	reverse_proxy nextcloud-nginx:80
 
 	header Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"