fix(usage notifications): handle plans with billing terms longer than 12 months

[matthewelwell]

Changes

This PR fixes an issue raised by a customer where they were sent a notification saying that they had used more than 500% of their allowance because the query to influx was returning all of their usage since the start of the billing period.

The bug was that relativedelta.months is not the same as relativedelta.total_months so actually the result from relativedelta.months was 0 rather than 12, because the relative delta looked something like relativedelta(years=1, months=0, ...).

I'm still unsure how we have billing terms that last for longer than 12 months, but this change should be robust to handle billing terms <12 months, or >12 months.

External (private) references for context:

Crisp chat with customer
Slack conversation

How did you test this code?

Added a new unit test.

[vercel]

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)			May 2, 2025 10:24am
flagsmith-frontend-preview	⬜️ Ignored (Inspect)			May 2, 2025 10:24am
flagsmith-frontend-staging	⬜️ Ignored (Inspect)			May 2, 2025 10:24am

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-5415	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-5415	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api-test:pr-5415	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-5415	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-5415	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-5415	Finished ✅	Results ✅

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.62%. Comparing base (d5b5e97) to head (9efdaad).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5415   +/-   ##
=======================================
  Coverage   97.62%   97.62%           
=======================================
  Files        1238     1238           
  Lines       43076    43088   +12     
=======================================
+ Hits        42053    42065   +12     
  Misses       1023     1023           

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