Build subscription proration engine with mid-cycle changes

[Smartdevs17]

Context

Implement build subscription proration engine with mid-cycle changes for the platform.

Current Limitation

This functionality is currently missing or needs improvement.

Expected Outcome

• Production-ready implementation
• Comprehensive test coverage
• Documentation and examples
• Performance benchmarks where applicable

Acceptance Criteria

• Feature implemented with full functionality
• Unit tests added with >80% coverage
• Integration tests for critical paths
• No regression introduced
• Documentation updated
• Code review approved
• Performance benchmarks met

Technical Scope

contracts/subscription/src/, backend/services/billing/proration.ts

Labels

• drips-wave
• high
• 200-points

[deedee-code]

@deedee-code has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi maintainer, i will like to work on this issue

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

[drips-wave]

Congratulations, @deedee-code! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @deedee-code: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊