Add configurable parallel downloads

[nittanygeek]

Summary

• add a configurable DOWNLOAD_CONCURRENCY setting for 1-4 simultaneous downloads
• change the sync worker from fixed batches to a rolling pool so slots refill as soon as a file finishes
• update live progress/session tracking so multiple active downloads render separately and ETA/speed stay sane

Testing

• PYTHONPATH=. .venv311/bin/python -m pytest tests/test_queue_ro_only.py tests/test_settings_schema.py tests/test_settings_api.py tests/test_sync_worker_cancel.py tests/test_sync_worker_enospc.py tests/test_sync_worker_concurrency.py tests/test_hub.py tests/test_hub_session_stats.py tests/test_download_session.py
• node --check web/static/app.js

Also tested manually against a Viofo dashcam with DOWNLOAD_CONCURRENCY=4, confirming four active downloads and continuous slot refilling.

[RobXYZ]

Thanks for the PR. Do you find this makes a noticeable speed increase for your downloads? I’m not seeing much difference on my A229 Pro which struggles with multiple connections. I often get the "slow card" message and I am wondering about the performance/thermal impact on the camera.