[Soroban] Add access control - only owner can mint

[Eniola3321]

Description:
Restrict minting so only the Clip owner (verified via backend signature or wallet) can mint their clips.
Acceptance Criteria:

Use Ownable pattern or custom modifier
Add only_clip_owner check using backend-provided signature
Prevent unauthorized minting

[Hallab7]

@Hallab7 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

i would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Hallab7 to this issue.