fix(#1036): eliminate billing-cycle date drift with date-fns

[alexatsejames-alt]

• Add weekly and annual cases to calculateNextRenewal using addWeeks/addYears
• Extend billing_cycle union type to include 'weekly' | 'annual' (was silently falling through to default, returning unchanged date on every renewal)
• Add property-style tests covering month-end clamping (Jan 31 -> Feb 28/29), quarterly end clamping (Mar 31 -> Jun 30), and no-drift assertions over 12 monthly and 52 weekly iterations
• Fix misleading test description 'add 365 days' -> calendar-aware addYears

Description

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Related Issue

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Test Plan

• Tested locally
• Verified expected behavior
• No regressions introduced

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Checklist

• Code builds successfully
• Tests pass
• Follows project conventions
• No sensitive data exposed
  Reminder billing-cycle date drift across intervals #1036

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#1036	Reminder billing-cycle date drift across intervals	Link to this issue

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