aria2 default --fsync=true calls fsync() after each write, which blocks
the entire single-threaded process on large files under Docker overlay.
Add --fsync=false to eliminate this blocking source.
Relax aiohttp session timeout: total=30 → sock_connect=10, sock_read=60
so that transient I/O delays don't cut off legitimate tellStatus RPCs.
Increase retry params (4 attempts, 3s delay) to give aria2 more recovery
time when blocked on synchronous I/O.
Root cause: aria2c subprocess stderr pipe (64 KB buffer) was never
drained. When enough error/warning output accumulated, aria2's write()
blocked, freezing the entire process including its RPC handler. The
tellStatus call then timed out after 30s with asyncio.TimeoutError(),
producing the empty error message in 'Failed to query aria2 download
status: '.
Fixes:
- Drain stderr in a background task so pipe never fills up
- Retry get_status() RPC calls up to 3 times on transient failure
- In the failure path, preserve .safetensors when .aria2 is absent
(the download was likely complete on disk)
