#68 API: List Available Roles

Implement endpoint to list all available roles for a tenant. Endpoint: GET /api/v1/roles Response: List of roles with their permissions Required Permissions: Any authenticated user (read-only) Implementation Requirements: - Return all 9 predefined roles - Include role description - Include list of permissions for each role - Include user count per role - Mark which roles user can assign (based on their role) Predefined Roles: - system_admin: Full system access - tenant_admin: Full tenant access - workflow_admin: Manage all workflows - workflow_operator: Execute and monitor - workflow_viewer: Read-only workflow access - developer: Create and edit workflows - analyst: View executions and reports - auditor: Access audit logs only - guest: Minimal read access With ultra attention to details and correctness. Before closing: write comment summarizing implementation with screenshot.