Contracts: Add storage pruning function to reclaim rent fees from expired data

[joelpeace48-cell]

Summary

On Soroban, storage that is not bumped expires and its rent fees are forfeited. The contract accumulates stale participant records, expired credits, and old nonces that bloat instance/persistent storage and increase minimum balance requirements. A pruning function cleans up expired data.

Acceptance Criteria

• Add prune_expired_participants(env, max_entries: u32) — removes participant records where TTL has passed (batch to avoid gas limits, max_entries cap per call)
• Add prune_used_nonces(env, admin, max_entries: u32) — removes nonce records older than 10,000 ledgers
• Both functions callable by anyone (no auth required — they only delete expired/stale data)
• Return the number of entries pruned
• Emit pruned event with count
• Add storage_stats(env) -> (participant_count, nonce_count, expired_estimate) view for monitoring
• Add unit tests: prune empty (no-op), prune with expired entries, max_entries cap respected

References

• contracts/campaign/src/lib.rs — participant storage
• contracts/rewards/src/lib.rs — nonce storage

[rahimatonize]

@rahimatonize has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, I would love to work on this issue. Could you please assign

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[dubemoyibe-star]

@dubemoyibe-star has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I'd like to work on this issue.

I'll implement the storage pruning functions for expired participant records and stale nonces, making them permissionless so anyone can help clean up unused storage. I'll also add the storage statistics view, emit pruning events, and include tests covering empty states, expired entries, and batch limits.

This should help reduce storage growth, reclaim rent costs, and make storage usage easier to monitor.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @dubemoyibe-star to this issue.

[Tobiloba0]

@Tobiloba0 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, let me work on this issue. Can finish it fast.

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[Abidoyesimze]

@Abidoyesimze has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi I'd like to work on this issue

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