feat: Add Security Inspector skeleton and results workflow

[nathanhuh]

Summary

Add the Security Inspector service shell, shared finding models, and the core TUI flow needed to run scans and browse findings.

Details

• Add ServiceInspector and FeatureSecurityScan constants in internal/domain/model.go
• Register Inspector in internal/domain/catalog.go
• Add finding and severity models in internal/services/aws/inspector_model.go
• Implement scan orchestration in internal/services/aws/inspector.go
• Add InspectorHome, InspectorScanning, InspectorResults, and InspectorFindingDetail screens
• Add severity filtering and empty/loading states in the Inspector UI
• Leave built-in rule packs to follow-up issues

Checklist

• Add Inspector service and feature constants
• Register Inspector in the catalog
• Add finding/severity models
• Implement scan runner orchestration
• Add Inspector screens and navigation flow
• Add severity filter support in results
• Add tests for screen transitions and scan orchestration

Related umbrella issue: #59

[nathanhuh]

@Unic-bot: assign me

[github-actions]

@nathanhuh has been assigned to this issue.