* feat(scrobbler): add exponential backoff delay helper
* feat(scrobbler): back off retries up to 4m during outages
* test(scrobbler): verify backoff schedule with synctest; clarify backoffDelay doc
Adds a testing/synctest-based test that drives the real run loop against a
failing service and asserts the exact 5s/10s/20s/40s retry schedule and the
drain-on-recovery reset, addressing the review note that the run loop's
behavior was untested. Also clarifies the backoffDelay doc comment: the
argument is a zero-based retry index.
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