Fix async file deletion bugs in assets endpoint (#5363) The delete handler had a missing `return` before `sendStatus(400)`, causing execution to fall through to `sendStatus(200)`, a double-send that triggers ERR_HTTP_HEADERS_SENT, which the catch block then compounds by attempting a third `sendStatus(500)`. Both the delete and download handlers used callback-based `fs.unlink()` without awaiting completion. In the download handler, this caused a race with `createWriteStream({ flags: 'wx' })` (which fails if the file still exists). In both handlers, `throw err` inside the callback was an unhandled exception that could never be caught by the outer try/catch. Replace callback-based `fs.unlink()` with `await fs.promises.unlink()` and add missing `return` statements to prevent response cascades.