fix: Set billing starts at to reasonable default for API usage notifications

[zachaysan]

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Changes

Set billing starts at to reasonable default for API usage notifications. This shouldn't be common in practice since most subscriptions set a billing period. Solves this Sentry issue.

How did you test this code?

N/A

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[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: api/organisations/views.py

Function	Unhandled Issue
get_queryset	TypeError: unsupported operand type(s) for +: 'relativedelta' and 'NoneType' /api/v1/organisations/{organisation... Event Count: 2

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[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.42%. Comparing base (9e2ffd2) to head (55130b6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4054   +/-   ##
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  Coverage   96.42%   96.42%           
=======================================
  Files        1146     1146           
  Lines       37386    37388    +2     
=======================================
+ Hits        36048    36050    +2     
  Misses       1338     1338           

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[matthewelwell]

This shouldn't be common in practice since most subscriptions set a billing period.

I don't think this is actually true since most subscriptions are on the 'free' plan.

[matthewelwell]

I'm going to approve and merge this for now because I think it's not relevant to organisations on the free plan, but we should discuss when you're around @zachaysan .