fix(storage): say why a repair cycle walked instead of reading the presence store

[rickyrombo]

Summary

blob_presence rows survive restarts. The decision to read them does not: every cycle re-runs presenceStoreReady, and any failed gate falls back to a full walk with the reason logged once at Debug. From outside, a node that spent a day enumerating and is doing it again after a restart looks the same whether the store is disabled, a bucket was never fully enumerated, or the liveness sample rejected the disk. That is what happened on audius.rickyrombo.com on 2026-09-10, and there was no way to tell which gate failed without shell access.

Changes

• presenceSourceForCycle now owns the per-cycle decision. It publishes a PresenceStoreStatus and returns whether the store is in use.
• Health check gains storage.presenceStore: {enabled, used, reason, checkedAt}. reason is the gate's error text ("archive has never been walked", "17 of 64 sampled keys missing under \"/archive-blobs\"", "cleanup cycle always enumerates its buckets", …), empty when the store is in use. Nil until the first cycle of the process decides.
• Warn instead of Debug when the store is enabled but unusable. With it off (the default) enumerating is the expected path and stays at Debug.
• dropFromBucket forgets the row it deletes. Archive eviction and relocation delete through it, and it only cleared the in-memory cache. A row left behind for a blob that is gone is exactly the drift the liveness sample exists to catch; enough of them would send every later cycle back to a full walk.

No behavior change to which path a cycle takes.

Follow-up

The console storage page shows the walk via storagev1.PresenceWalk. Surfacing reason there too needs a proto field and regeneration, so it is left for a separate PR.

Test plan

• make test-mediorum passes
• TestPresenceSourceForCycleReportsWhyItWalked: disabled, cleanup, never-walked, and ready each publish the expected used/reason
• TestDropFromBucketForgetsPresence: a bucket-scoped delete removes the store row

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