feat: implement direct R2 uploads via presigned URLs

Commit notes

New upload flow: 1. Browser requests presigned URLs from server for all chunks 2. Browser uploads directly to R2 using presigned URLs 3. Browser confirms each chunk with server 4. Falls back to server upload if CORS blocks direct access

Benefits: - Server only handles metadata, not file data - Reduces server CPU/memory/bandwidth - Faster uploads (direct to storage) - Automatic fallback ensures reliability

Co-authored-by: armin.naimi <[email redacted]>