Recomputes the MD5 checksums the upload API used to record (G12), as a job run
(storage.checksum_backfill): a file stored with a 32-hex-character checksum has
its original read and its SHA-256 and matching user_file_checksum recorded, twenty
a batch, in uuid order, five seconds apart on the file_processing queue.
The work of a batch is PhoenixKit.Modules.Storage.Workers.ChecksumBackfillJob's
(run_batch/2). Started after boot and by the daily trash prune while any MD5 row
is left (ChecksumBackfillJob.maybe_enqueue/0).
done counts the files handled (updated, left because their uploader has the bytes
already, or edited meanwhile — all final), failed those whose original could not
be read (marked, so a later pass does not download them again). Replaying a batch is
safe: the update is guarded on the old checksum. restart/0 is :merge.