[Ayinkx]

Closes #443

Summary

• Add explanatory tooltips to estimated-cost totals and table values in billing and admin usage reports.
• Make the estimate disclaimer available to assistive technology on the totals.
• Add a regression test asserting the admin report exposes the estimate label and tooltip.

Validation

• Added a targeted pytest regression test.
• CI must run on this PR; I have not claimed local test results.