Implement `POST /donations` record donation endpoint

[ayshadogo]

User Story:
As a donor, I want my donation recorded on the backend after a successful blockchain transaction so that it appears in campaign stats.
Acceptance Criteria:

• Accepts: campaignId, txHash, amount, assetCode, assetIssuer, isAnonymous
• Verifies transaction on Stellar Horizon before recording
• Prevents duplicate recording (idempotent on txHash)
• Returns created donation record
• Triggers campaign stats recalculation

---

[Power70]

@Power70 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintainer,

I can work on this issue

Kindly assigned it to me

Thank you

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Power70 to this issue.

[jonathanayubausara-a11y]

@jonathanayubausara-a11y has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

i would like to work on this issue because
i can improve the recording system

ℹ️ Repo Maintainers: To accept this application, review their application or assign @jonathanayubausara-a11y to this issue.

[Cedarich]

@Cedarich has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello maintainer can i take up this task

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

[Amas-01]

@Amas-01 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello Maintainer
I'll like to work on this Issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Amas-01 to this issue.

[drips-wave]

Congratulations, @Cedarich! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @Cedarich: 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.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[drips-wave]

This issue has been completed by @Cedarich as part of the Stellar Wave Program's 5th Wave 🥳

😎 @Cedarich: 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.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.