#14075 feat(dashboard): metered tier editor and display

Summary

Villiam's work from #14041, adapted to the split metered price type.

Stacked on #14072. Supersedes #14041.

https://github.com/user-attachments/assets/d88e13b1-5f53-4cc0-9481-926eaf46080c

https://github.com/user-attachments/assets/4b442f69-e4bd-4c1e-a304-676d8ad15b04

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

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}