feat: improve API integration and referral attribution

[aizyohiy-hash]

Summary

This PR introduces two enhancements aimed at improving the developer experience and platform capabilities by expanding API documentation and adding referral attribution support for contributions.

Implemented

• Documented API error codes in the OpenAPI specification to provide integrators with clear, standardized error responses.
• Added referral attribution tracking to contributions, enabling referral relationships to be recorded and analyzed.

Goals

• Improve the API integration experience with comprehensive error documentation.
• Reduce ambiguity for developers consuming the API.
• Enable accurate attribution of referred contributions for analytics and future incentive programs.
• Strengthen the platform's extensibility and observability.

Type of Change

• Enhancement
• Bug Fix
• New Feature
• Breaking Change
• Documentation Update

Notes

These enhancements improve platform usability and integration without introducing breaking API changes in prog

Closes #338
Closes #346