Migrate stacks/database.go from survey to huh

[blsmth]

Summary

Migrate database stack prompts in stacks/database.go from survey to huh.

Details

• Complexity: Medium
• Prompts: 4 Selects (engine, aurora variant, instance class, multi-az)
• Conditional flow — engine + aurora prompts only on new creation
• Aurora choice transforms engine name
• Instance classes fetched from AWS RDS API
• Depends on AskForCluster migration (stacks/stacks.go issue)

Dependencies

• Requires AskForCluster migration to be completed first

Checklist

• Extract form builder functions (pure, no AWS mocks needed)
• Replace ui.AskQuestions calls with huh forms
• Handle conditional engine/aurora flow cleanly
• Add teatest + ui/uitest tests
• Add VHS recording for visual verification
• Verify non-interactive mode still works