sourcemaps upload sends files one at a time #860

Is your feature request related to a problem? Please describe.
ldcli sourcemaps upload PUTs each file to its presigned URL in a sequential loop. Next.js server output has thousands of files, so the upload is dominated by request round trips. For our Next.js 16 app (3,474 files, 206 MB from .next/server), the upload takes 2 minutes inside our Vercel build and 6.5 minutes from a laptop.

Describe the solution you'd like
Upload a bounded number of files concurrently. get_source_map_upload_urls_ld already returns every presigned URL in one call, so only the PUT loop changes.

Describe alternatives you've considered

• Running several ldcli processes on subdirectories: works, but needs a split per app and loses the single success/failure result.
• Calling the GraphQL endpoint directly with our own uploader: it is undocumented, so we would rather fix the CLI.

Additional context
Measured from a laptop with the patch from #861 (2 runs each, same build output):