fix(overage-billing): use relative delta and months, instead of days

[matthewelwell]

Changes

Since a billing period for anyone on a monthly billing cycle is a month, not 30 days, we should use a month to determine the period in which we're looking for previous usage notifications.

How did you test this code?

Updated an existing unit test.

[vercel]

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		Feb 11, 2025 11:31am
flagsmith-frontend-preview	⬜️ Ignored (Inspect)	Visit Preview		Feb 11, 2025 11:31am
flagsmith-frontend-staging	⬜️ Ignored (Inspect)	Visit Preview		Feb 11, 2025 11:31am

[github-actions]

Uffizzi Preview deployment-60455 was deleted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.43%. Comparing base (3f7c9da) to head (82b6967).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5060   +/-   ##
=======================================
  Coverage   97.43%   97.43%           
=======================================
  Files        1214     1214           
  Lines       42308    42312    +4     
=======================================
+ Hits        41224    41228    +4     
  Misses       1084     1084           

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[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-5060	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-5060	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-5060	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-5060	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-5060	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-5060	Finished ✅	Results ✅