Fix cache headers: immutable for hashed assets, revalidate for HTML

The blanket max-age=3600 caused stale HTML pages after redeployment.
Now _astro/* gets immutable (content-addressed filenames) and HTML
pages get no-cache (always revalidate, 304 when unchanged).

docs/Caddyfile

@@ -2,6 +2,14 @@
 	root * /srv
 	file_server
 	encode gzip zstd
-	header Cache-Control "public, max-age=3600"
+
+	# Hashed assets: cache forever (filename changes on content change)
+	@hashed path /_astro/*
+	header @hashed Cache-Control "public, max-age=31536000, immutable"
+
+	# Everything else (HTML pages): always revalidate
+	@unhashed not path /_astro/*
+	header @unhashed Cache-Control "no-cache"
+
 	try_files {path} {path}/ /404.html
 }