[Documentation] Create frontend-backend API contract document

[Eniola3321]

Labels: documentation, api
Description:
Frontend and backend teams need a clear contract for data exchange.
Acceptance Criteria:

Create docs/api-contract.md
Document request/response shapes for major endpoints
Include examples for clip curation and NFT mint
Difficulty: beginner

[WHIZAB4TECH]

I’m interested in contributing to this issue and believe I can deliver a clean, well-tested solution that aligns with the project’s standards.