Add multi-signature support for admin operations

[abayomicornelius]

Problem

Single admin key controls all contracts. Compromise or loss of key is catastrophic.

Why it matters

Production needs multi-party governance for security.

Suggested fix

Support Stellar native multi-sig or implement m-of-n admin verification.

Acceptance criteria

• Design multi-sig admin structure
• Update admin checks across contracts
• Add multi-sig deployment guide
• Test multi-sig scenarios

[adelekevictor12]

@adelekevictor12 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello maintainer, i can fix this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @adelekevictor12 to this issue.