hotfix(backend): Fix month credit calculation on December

[majdyz]

When calculating the next month, we are not rolling the month number causing an error on credits.

Changes 🏗️

Add modulo while calculating next month.

Checklist 📋

For code changes:

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[qodo-merge-pro]

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🧪 No relevant tests

🔒 No security concerns identified

⚡ Recommended focus areas for review Edge Case Verify that the month rollover logic works correctly when transitioning from December to January, including handling of the year change

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[majdyz]

I'm going to merge this directly since it breaks the CI & the running system.