Rename express/ to backend/ and update references

Update paths in sync.sh, master/main.go, and CLAUDE.md to reflect
the directory rename.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/build.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+set -eu
+
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+cd "$DIR"
+
+../cmd pnpm ncc build ./app.ts -o dist