Backup encryption and improvements

ts/util/prependStream.ts

@@ -0,0 +1,17 @@
+// Copyright 2024 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { Transform } from 'stream';
+import type { Duplex } from 'stream';
+
+export function prependStream(data: Uint8Array): Duplex {
+  return new Transform({
+    construct(callback) {
+      this.push(data);
+      callback();
+    },
+    transform(chunk, _encoding, callback) {
+      callback(null, chunk);
+    },
+  });
+}