The playlist ExternalImageURL step used sources.go's fromURL, which maps any
non-200 to a generic error, so a stale URL returning 404/410 was classified
transient: infinite backoff plus it counted toward the circuit breaker,
blocking valid external work. Add a local fetch in resolve.go that maps
404/410 to model.ErrNotFound (definitive) while keeping other non-200s
transient. sources.go is left untouched.
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