Add transaction deduplication using idempotency keys

[sshdopey]

What

The admin endpoint and cron job may submit duplicate Stellar transactions if retries trigger or the endpoint is called multiple times quickly. There is no deduplication mechanism.

Why

Duplicate transactions waste XLM on fees and may cause unexpected on-chain state changes.

Scope

• Generate a unique idempotency key per score update
• Store recent keys in memory with TTL
• Reject duplicate submissions within the TTL window
• Make the TTL configurable

Acceptance Criteria

• Each updateImpactScore call generates a unique key
• Duplicate keys within IDEMPOTENCY_TTL_MS are rejected
• Key is based on projectId + hourSeed (deterministic per project per hour)
• In-memory store with configurable TTL
• Logs duplicate attempts

Technical Context

• src/lib/registry.ts:18-41 — updateImpactScore function
• src/routes/admin.ts:41-53 — admin loop
• src/index.ts:29-38 — cron loop
• Key formula: score:${projectId}:${hourSeed}

[esegbueadam-sys]

@esegbueadam-sys has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi, maintainer. I Would like to work on the issue.

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

[omoh25]

@omoh25 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I would like to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @omoh25 to this issue.

[drips-wave]

Congratulations, @omoh25! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @omoh25: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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