Against the fake Ente server, with each download held until the test lets
it go: a library with the default pools has five downloads in flight and a
pool of two has two; with verify, only the originals whose downloads are
in flight have been hashed; a 404 for one photo does not stop the rest;
and two runs end the same with a pool of one as with the default pools.
Today the backup downloads one original at a time, so the first four fail.
Model: opus-5-5