fix(server): dedupe database backup jobs (#28341)

* fix(server): dedupe database backup jobs via jobId

#27268 shows backup jobs piling up in the queue across upgrades; one pending
backup is always enough.

* fix(tests): Avoid stale backup files from previous test runs being erroneously returned from createBackup

* fix(jobs): Use bullmq's deduplication over jobId to avoid failed jobs from blocking future executions.

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Co-authored-by: Robert Deaton <immich@rdeaton.space>