feat: build subscription proration engine with mid-cycle changes

[senmalong]

Summary

Implements a production-ready subscription proration engine for mid-cycle plan upgrades, downgrades, and billing cycle conversions across both backend billing services (backend/services/billing/proration.ts) and Soroban smart contracts (contracts/subscription/src/).

Key Changes

• Backend Billing Service (backend/services/billing/proration.ts):
  • Implemented ProrationEngine class supporting calculateMidCycleProration, generateCreditMemo, applyCreditBalance, and calculateNetProration.
  • Added support for exact second/day ratio proration calculations, immediate vs end-of-period effective dates, and cross-interval conversions.
• Smart Contracts (contracts/subscription/src/):
  • Verified and integrated on-chain calculate_proration, preview_proration, and change_plan contract methods.
• Documentation:
  • Added docs/PRORATION_ENGINE.md documenting proration formulas, credit memo lifecycle, and billing API examples.

Closes #959

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[Smartdevs17]

Thanks for contributing! The changes have been merged. Feel free to leave a review or feedback.