feat: implement high-performance resumable upload system

Commit notes

Major overhaul of the upload system with:

Backend: - Added presigned URL endpoints for direct S3 uploads - Added chunk confirmation endpoint for presigned uploads - Increased rate limits (100 -> 1000 chunks/min) - Increased concurrent upload limit (10 -> 50) - Increased batch initiate limit (50 -> 100 files) - Added adaptive chunk sizing based on file size - Support for custom chunk sizes from client

Frontend: - New upload engine with 8+ concurrent chunk uploads - IndexedDB persistence for cross-session resumability - Direct S3 uploads via presigned URLs (bypasses server) - Adaptive chunk sizing (512KB-5MB based on file size) - Real-time speed metrics (current, peak, average) - Pause/resume/cancel controls - Resume prompt for pending uploads on page load - Improved progress panel with speed display

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