Implement campaign close endpoint

[ayshadogo]

User Story:
As a creator, I want to close my campaign early so that donors know fundraising has ended.
Acceptance Criteria:

• POST /campaigns/:id/close sets status to CLOSED
• Only creator can close their own campaign
• Requires wallet signature verification (replay of signed payload)
• Triggers notification to all donors
• Records close event in AuditLog

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[RaymondAbiola]

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

Hi, I’d like to work on this issue. I have experience with rust, Solidity, frontend, and backend development, and I’m confident I can deliver a clean solution.

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

[feyishola]

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

Hi im Feyishola a software developer with over 4 years of experience. Please let me handle this issue

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

[drips-wave]

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

🧑‍💻 @feyishola: 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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