draft: feat: Status page Slots modal

[royendo]

In order to support new pricing model, https://rilldata.slack.com/archives/C0ADZ6Q3A91/p1773346718712579

Admins can upgrade slots on their own without requesting Customer Support

TODO:

• Add unique behavior based on Plan
  • Free, view only
  • Growth, Variable
  • Enterprise, custom

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!