A truncated debrid download (in-memory byte counter hit 100% while the
NFS write-back silently dropped most of the bytes) was marked completed.
The 1.1.6 fsync fix closed the debrid-specific hole; this generalizes the
guarantee so "completed" never means a corrupt file on ANY backend.
- IntegrityError + bounded retry: on a corrupt/short result the manager
re-downloads the same source up to 3x (clean start), then surfaces the
task as damaged ("corrupt download:" prefix) instead of completing it.
- verify (size mismatch / empty), debrid (incomplete / post-write / flush),
torrent (BytesMissing), usenet (par2 unrepairable / repair-failed) all
classify integrity failures so they route through the retry/damaged path.
- scanner: a file ffprobe can't read is emitted as a damaged library_item
(reason "unreadable") instead of being silently dropped from the sync.
- tests: manager retry-then-success + retry-exhausted-then-damaged,
verifying->resolving transition, damaged sync item.
