feat: Render usage report in sales dashboard usage directly, without sending email

[rolodato]

Thanks for submitting a PR! Please check the boxes below:

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

Previously, we were requiring users to enter an email address so that they could receive these reports:

This is not ideal for several reasons:

• Not all self-hosted customers have an email provider set up. This change lets these customers access reports without needing to set up email.
• Even if all customers did have email providers, email is a generally awful medium to work with. HTML, PDF, etc are much easier to use and test, for both us and customers.
• The email report was synchronously generated and sent as part of the same request; we are not sacrificing performance by returning the output via email or HTML.

This change also intentionally breaks the /sales-dashboard/email-usage route by renaming it to /sales-dashboard/usage. It is extremely unlikely that this will actually break anyone's links, considering it's not a documented route and this route would be used ~2x a year at most.

Alternatives considered

If we really want to keep emails for some reason, we could have a plain-text report be sent directly from the client, by using a mailto link from the same HTML-rendered report. I decided to keep HTML only for simplicity.

How did you test this code?

Manually by browsing to /sales-dashboard/usage on any API instance.

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		May 5, 2025 2:53pm
flagsmith-frontend-preview	⬜️ Ignored (Inspect)	Visit Preview		May 5, 2025 2:53pm
flagsmith-frontend-staging	⬜️ Ignored (Inspect)	Visit Preview		May 5, 2025 2:53pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-5407	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-5407	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-5407	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-5407	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-5407	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-5407	Finished ✅	Results ✅

[github-actions]

Uffizzi Preview deployment-63319 was deleted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.63%. Comparing base (705560b) to head (b0e23be).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5407      +/-   ##
==========================================
+ Coverage   97.62%   97.63%   +0.01%     
==========================================
  Files        1238     1239       +1     
  Lines       43037    43065      +28     
==========================================
+ Hits        42014    42046      +32     
+ Misses       1023     1019       -4     

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[matthewelwell]

Happy for you to add # pragma: no cover where needed if it's easier than writing tests to cover the view.