Implement contract ownership check helper

[ayshadogo]

User Story:
As a developer, I want a reusable authorization helper so that creator-only functions are consistently protected.
Acceptance Criteria:

• fn require_creator(env: &Env) helper function
• Reads creator from storage
• Calls creator.require_auth()
• Panics with Error::Unauthorized if caller is not creator
• Used in all creator-only functions

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[Simongodw]

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

greetings maintainer, I hope you are having a great day. I’m interested in working on this issue and would be thankful if you could assign it to me.

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

[portableDD]

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

Can i work on this?

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

[drips-wave]

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

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