Feature/billing cycle fix 142

[Robinsonchiziterem]

PR #142 Billing cycle calculation uses fixed 30/90/365 day intervals — drifts for calendar month subscriptions

Description

This PR standardizes the billing cycle calculations across the SYNCRO backend and client library. Previously, the system used fixed-day arithmetic (30/90/365 days) which caused date drift over time (e.g., January 31st projecting to March 2nd/3rd instead of February 28th/29th).

The new implementation uses date-fns calendar-aware functions (addMonths, addQuarters, addYears) to ensure accurate renewal dates that respect varying month lengths and leap years.

Key Changes:

• Simulation Service: Refactored calculateNextRenewal to use date-fns.
• Renewal Executor: Standardized next billing date calculation during the renewal process.
• Client Utils: Updated subscription-utils.ts to use addDays consistency.
• Dependencies: Added date-fns to the backend.

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Test Plan

• Tested locally (verified with unit tests for Jan 31st and Feb 29th transitions)
• Verified expected behavior (projections correctly align with calendar months)
• No regressions introduced (all existing logic and mathematical averages in analytics remain consistent)

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Screenshots (if applicable)

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Checklist

• Code builds successfully
• Tests pass
• Follows project conventions
• No sensitive data exposed

Closes #142

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