server: add workload-adaptive auto-periodic compaction mode

[evanphx]

Add a third --auto-compaction-mode, "auto-periodic", alongside "periodic" and "revision". Rather than a fixed schedule, it samples the backend's growth rate and times each compaction so the projected size stays below a fraction of --quota-backend-bytes, adapting to the workload: faster growth compacts sooner (more headroom exactly when needed), an idle store backs off. It keeps the last --auto-compaction-retention revisions, like the revision mode.

Like the periodic and revision modes it only compacts and never defrags, so reclaiming physical disk space remains a separate, operator-initiated operation.

The mode needs the backend size and the effective quota, so v3compactor.New gains a SizeGetter and a maxBytes argument; the periodic and revision compactors ignore them.

This change was written with the assistance of Claude.

[kubernetes-prow]

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[kubernetes-prow]

Hi @evanphx. Thanks for your PR.

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