Storage pruning jobs for expired nonces, snapshots, and stale indices

[joelpeace48-cell]

Epic: Notifications & Messaging

Summary

Ensure every growing on-chain index (nonces, snapshots, referral/participant indices) has a capped, callable pruning path.

Background & Problem

Without pruning, auxiliary indices accumulate and reintroduce the instance-storage size risk over time.

Acceptance Criteria

• Each growing index has a capped prune function.
• Pruning is permissionless where safe.
• Tested at scale.

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Filed as part of the mainnet-readiness & world-class roadmap. Scope is intentionally small enough for one focused PR; comment to claim.