Adds RETRY_ATTEMPTS (3..=5, default 3) and RETRY_BACKOFF_MS
(100..=30000, default 1000) to Settings, validated with the same
panic-on-invalid contract as POOLING and CHUNK_SIZE_MB.
The combinator logs every failed attempt and any late success through
the JobLogger it borrows, so retries reach the server on the existing
job-log path with no API change. It borrows rather than clones the Arc
so Arc::try_unwrap in the backup executor keeps working. Backoff is
exponential with equal jitter, because the uploader retries storages
concurrently and would otherwise retry them in lockstep.
charles-gauthereau