simplify: remove direct R2 upload, keep server-based approach

Commit notes

Removed direct upload complexity. The speed gains come from: - Rate limit: 100 → 1000 chunks/min (10x) - Chunk concurrency: 4 → 8 (2x) - File concurrency: 6 → 8 (1.3x) - Adaptive chunk sizing (backend)

Expected improvement: 25 min → ~8-10 min for 650 files / 1.52GB

Backend presigned URL endpoints kept for future use if needed.

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