feat: log permanently failed downloads to failures.json

State advances past a download that exhausts MAX_RETRIES so one broken
file cannot wedge the backup, which until now left a silent gap in the
archive. Failures are appended to failures.json in the archive dir as
JSONL, one {id, file, error} per line; appending is a single write, so a
crash costs the last record rather than the file.

download_with_retry now returns the last exception or None on success,
replacing the True/False return, so the caller has an error to log.

Nothing reads or prunes the file: it is a record, not a retry queue.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions