Add paid-query settlement audit digest endpoint

[emrekayat]

Why this matters

SCF reviewers and investor diligence need a compact way to verify that paid-query activity is backed by recorded payment evidence, not just dashboard totals.

Scope

Add a read-only API endpoint that returns a small audit digest for recent paid queries:

• total paid runs counted;
• total settled amount by asset/network;
• number of records with payment evidence present;
• number of records missing evidence;
• latest recorded payment timestamp;
• deterministic generatedAt timestamp.

Suggested route:

• GET /api/audit/settlement-digest

Suggested files:

• apps/api/src/routes/public.ts
• apps/api/src/lib/persistence.ts
• packages/shared/src/schemas.ts

Acceptance criteria

• Endpoint is read-only and does not trigger provider execution or payment.
• Empty analytics storage returns zeros, not a 500.
• Demo-mode and real-payment rows are distinguishable if that metadata already exists.
• Unit or API test covers empty storage and at least one populated digest.
• Existing npm test --workspace @query402/api still passes.

Wave complexity

High - reviewer/investor evidence surface for paid-query settlement health.

[Tobi-8]

@Tobi-8 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hey maintainer, I have experience in handling this issue
Kindly assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Tobi-8 to this issue.

[samjay8]

@samjay8 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello Boss, I will like to work on this.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @samjay8 to this issue.

[drips-wave]

Congratulations, @samjay8! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @samjay8: 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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