verify: match USDC asset issuer on Horizon payment

[CemAyyildiz]

Goal

Extend the MVP verify path so credit-asset payments must match both invoice.assetCode and invoice.assetIssuer. Native XLM invoices must still require asset_type === 'native'.

Context

docs/ASSETS.md requires USDC invoices to be identified by code plus issuer. Today backend/src/server-mvp.ts only compares asset_code, and backend/src/utils/verify-invoice-payment.ts has no issuer field. A payment in a different testnet USDC token with the same code could be accepted as settlement.

Related but separate: #457 covers validation tests for USDC + issuer on create, not Horizon verify matching.

Acceptance criteria

• verify-invoice-payment.ts accepts optional issuer fields and rejects credit payments when issuers differ
• server-mvp.ts verify handler passes paymentOp.asset_issuer and compares against invoice.assetIssuer for non-native assets
• XLM verify behavior unchanged
• Unit tests in verify-invoice-payment.test.ts: XLM match, USDC code+issuer match, wrong issuer → ASSET_MISMATCH
• docs/VERIFY.md documents issuer checks

Out of scope

• Shared invoice API contract types
• Proof PDF / quittance output changes
• Automatic payment detection / streaming
• Mock API retirement
• Invoice expiry enforcement

Conflict rule

• Do not edit: frontend/lib/export.ts, frontend/lib/mock-api.ts, frontend/app/pay/[id]/page.tsx

Complexity

Complexity: Medium — 150 points (Stellar Wave)

[MrOmale]

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

I will love to work on this issue please. I deliver high impact solutions, ensuring seamless workflow and client satisfaction

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[oluwafunmilayo-coder]

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

Hi, maintainer can i be assigned this issue pleasee?

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[kalebosas2-dev]

@kalebosas2-dev has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I would love to work on this please

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[cythecode]

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

Hello maintainer, I’ve got this covered! Ready to deliver a clean, high-impact PR—swiftly done and ready to get merged seamlessly

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[SamixYasuke]

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

Hello Maintainer,

I can extend the verification flow to validate both the USDC asset code and issuer while preserving the existing native XLM behavior. I’ll update the payment verification, Horizon handler, focused unit tests, and verification documentation, while respecting the listed out-of-scope files.

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[Muyideen-js]

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

Hi, I can fix this

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[egekoca]

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

please sir

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[drips-wave]

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

🧑‍💻 @egekoca: 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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