placeholder for caching - Always returns a random ETag header.

So the browser-side cache will always miss, but I tested that Firefox will at
least send us If-None-Match with the last ETag it saw.

crates/ptth_server/src/file_server/mod.rs

@@ -189,6 +189,21 @@ async fn serve_file (
 	
 	let mut response = Response::default ();
 	
+	// The cache-related headers in HTTP have bad names. See here:
+	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
+	// The intended semantics I'm using are:
+	// - etag - Some random hashed value that changes whenever the metadata
+	//   (name, inode number, length, mtime) of a file changes. Also changes
+	//   on new server instance.
+	// - no-cache - Clients and the relay can store this, but should revalidate
+	//   with the origin server (us) because only we can check if the file
+	//   changed on disk.
+	// - max-age=0 - The file might change at any point during or after the
+	//   request, so for proper invalidation, the client should immediately
+	//   consider it stale.
+	
+	response.header ("cache-control".to_string (), b"no-cache,max-age=0".to_vec ());
+	response.header ("etag".to_string (), rusty_ulid::generate_ulid_string ().into_bytes ());
 	response.header (String::from ("accept-ranges"), b"bytes".to_vec ());
 	
 	if range_requested {