#14110 feat(checkout): tiered metered pricing display

Summary

Supersedes @strandhvilliam's #14042, rewritten against the current backend.

Graduated, collapsed. The default. From $12.00 / unit under the price, detail rows behind the #13945 toggle.

Graduated, expanded. One row per tier. Bounds are inclusive, so a tier starts one unit past the one below it. The meter line drops its rate: repeating $12.00 directly above the row that already states it reads as two different prices.

Volume, expanded. Same table plus one caption.

50% discount. Every tier rate struck through and restated. The price line and the tier rows render rates through one component, so a discount cannot apply to one and not the other.

Flat metered_unit price, unchanged.

Checklist

• This PR addresses a single concern (one bug fix, one feature, one refactor)
• The diff is reasonably sized and easy to review
• New functionality is covered by tests
• Linting and type checking pass (uv run task lint && uv run task lint_types)
• No unrelated changes or drive-by fixes are included