fix: optimise influxdb calls from handle_api_usage_notifications task

[matthewelwell]

Changes

Optimises the handle_api_usage_notifications task in 2 ways:

1. Only run the logic for organisations with a subscription cache record
2. Only request data from influx for organisations that have exceeded the minimum notification threshold in the last 30 days

How did you test this code?

Updated existing tests

[vercel]

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3 Skipped Deployments

Project	Deployment	Review	Updated (UTC)
docs	Ignored	Preview	Jan 2, 2026 11:53pm
flagsmith-frontend-preview	Ignored	Preview	Jan 2, 2026 11:53pm
flagsmith-frontend-staging	Ignored	Preview	Jan 2, 2026 11:53pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-6458	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-6458	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-6458	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-6458	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6458	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6458	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.07%. Comparing base (38ac162) to head (9e6ec99).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6458   +/-   ##
=======================================
  Coverage   98.07%   98.07%           
=======================================
  Files        1294     1294           
  Lines       46535    46537    +2     
=======================================
+ Hits        45637    45639    +2     
  Misses        898      898           

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[emyller]

LGTM