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>

sync.sh

@@ -79,5 +79,5 @@ cd "$DIR/logger"
 go build
 
 # Update framework code
-cd "$DIR/express"
+cd "$DIR/backend"
 ../cmd pnpm install