Add integration test suite for the full payment-to-settlement flow

[therealjhay]

Current behavior:
Unit tests exist but no end-to-end test spanning merchant → payment → settlement → webhook.

Expected behavior:
Write integration test using test PostgreSQL and Redis (testcontainers or docker-compose). Flow: create merchant, initiate payment, complete payment, trigger settlement, verify settlement processed with correct fees, simulate webhook delivery. Mock Stellar RPC and CoinGecko. Run in CI.

Files to modify:

• New test file: services/api-gateway/tests/integration/payment-to-settlement.test.ts
• CI configuration

Test requirements:
Single test covers the full flow with assertions at each step.

Acceptance criteria:

• End-to-end integration test passes in CI.
• Covers the critical payment-to-settlement path.

[sunnykid-02]

@sunnykid-02 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I’d like to work on this issue because an end-to-end integration test for the complete payment-to-settlement flow is essential for validating one of the system’s most critical business workflows. Covering the entire lifecycle—from payment initiation through processing and final settlement—helps detect integration issues that unit tests cannot, while providing strong protection against regressions.

I have experience writing integration tests for backend services, payment systems, and event-driven workflows.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @sunnykid-02 to this issue.

[Oyinkans0la12]

@Oyinkans0la12 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Can i work on this please?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Oyinkans0la12 to this issue.

[Caneryy]

@Caneryy has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello, can I work on this issue?

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

[drips-wave]

Congratulations, @Oyinkans0la12! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @Oyinkans0la12: 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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[drips-wave]

This issue has been completed by @Oyinkans0la12 as part of the Stellar Wave Program's 7th Wave 🥳

😎 @Oyinkans0la12: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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