TODO-052/S56 design 3C: a per-worker-process concurrency cap
(OTA_MAX_CONCURRENT_DOWNLOADS, 503 download_busy + Retry-After: 15,
acquired BEFORE the deployment status flips to downloading and
released only once the response body has actually finished streaming —
see app.services.download_guardrails), plus HTTP Range support (206 +
Content-Range + Accept-Ranges — the installed Starlette FileResponse does
not support Range at all, verified by reading its source). A
Range-resumed request (start > 0) does NOT re-flip deployment
status/progress — only a fresh download (no Range, or Range starting at
byte 0) does.