limit Caddy compression to text content types to fix slow file downloads

Caddy was compressing all responses including binary file downloads
(PDFs, images, videos), which severely throttled download speed to
~130KB/s despite 30MB/s VPS bandwidth. Now only compresses text-based
types (HTML, CSS, JS, JSON, XML, SVG) where compression actually helps.

caddy/Caddyfile

@@ -9,7 +9,17 @@
 }
 
 nextcloud.t-gstone.de {
-	#encode zstd gzip
+	encode zstd gzip {
+		match {
+			header Content-Type text/*
+			header Content-Type application/javascript*
+			header Content-Type application/json*
+			header Content-Type application/xml*
+			header Content-Type application/rss*
+			header Content-Type application/atom*
+			header Content-Type image/svg*
+		}
+	}
 
 	reverse_proxy nextcloud-nginx:80