feat(saas-billing): Invoice immediately for new seats

[matthewelwell]

Changes

Resolves https://github.com/Flagsmith/flagsmith-private/issues/108

• Invoices scale-up customers immediately for new seats instead of pro-rating to the next billing cycle.
• Uses a new addon which ensures that addons are billed according to the correct billing frequency

How did you test this code?

Existing unit tests should cover the behaviour, but I will also test manually in staging.

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored		Mar 31, 2026 5:34pm
flagsmith-frontend-preview	Ignored		Mar 31, 2026 5:34pm
flagsmith-frontend-staging	Ignored		Mar 31, 2026 5:34pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-7086	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-7086	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-7086	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-7086	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-7086	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-7086	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (3caa120) to head (a7885eb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7086   +/-   ##
=======================================
  Coverage   98.33%   98.33%           
=======================================
  Files        1335     1335           
  Lines       49943    49943           
=======================================
  Hits        49113    49113           
  Misses        830      830           

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[github-actions]

Playwright Test Results (oss - depot-ubuntu-latest-16)

 10 passed

Details

 10 tests across 7 suites
 42.1 seconds
 a7885eb
 🔄 Run: #15596 (attempt 1)

Playwright Test Results (oss - depot-ubuntu-latest-arm-16)

 10 passed

Details

 10 tests across 7 suites
 42.9 seconds
 a7885eb
 🔄 Run: #15596 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-16)

 1 passed

Details

 1 test across 1 suite
 43.2 seconds
 a7885eb
 🔄 Run: #15596 (attempt 1)

Playwright Test Results (private-cloud - depot-ubuntu-latest-arm-16)

 16 passed

Details

 16 tests across 13 suites
 54.3 seconds
 a7885eb
 🔄 Run: #15596 (attempt 1)