Add Evidence CID Schema

[Naomi-Gift]

Verification evidence CIDs should point to structured evidence, but there is no schema.
Acceptance criteria:
Define verification evidence schema.
Include proof links, signatures, screenshots, or attestations where appropriate.
Add example evidence document.
Document privacy and safety expectations.

[Tobi-8]

please maintainer , assign me to this issue.
Hey maintainer, i have looked through your description, and i have had experience in this type of issue.
kindly assign

[clintjeff2]

Hello, I have 6 years of experience working on tasks like this and handling similar challenges across backend, frontend, and smart-contract development. I have carefully read and fully understood the task requirements, and I am confident I can deliver a clean, efficient, and well-tested solution promptly. I always ensure high-quality implementation, maintainability, and adherence to best practices.

Pull request in 5 hours.