Replace SAM2 with geometric ellipses for anatomy regions

SAM2: pick mask by highest IoU within size range, not smallest

Add bbox prompts alongside click points for SAM2

Fix SAM2 mask selection + pin MediaPipe to keep solutions API

Make SAM2 mask indexing shape-agnostic

Drop reshaped_input_sizes arg from post_process_masks

Fix SAM2 processor 4-level input + adjusted output indexing

Add per-file download progress + elapsed-time logging to SAM2 init

Add /api/logs endpoint and per-step print logging for SAM2 progress

Initial backend