A failed aria2 transfer deleted the partial payload while keeping its
.aria2 control file, and "No URI available" (expired CivitAI signed URL)
was treated as a permanent failure, wasting nearly-complete downloads.
- Re-schedule the transfer with a freshly resolved signed URL and
continue=true when aria2 reports "No URI available", bounded by
MAX_TRANSFER_RECOVERY_ATTEMPTS
- Keep payload and .aria2 control file together as a resumable pair
after a failed transfer instead of deleting the payload
- Report and remove orphaned .aria2 control files that have no payload,
both after failures and when restoring persisted downloads
Fixes#1088