#258 Implement super admin tenant management APIs

Create tenant CRUD endpoints for super admin users to manage customer tenants. Required endpoints: - POST /api/v1/admin/tenants - Create tenant - GET /api/v1/admin/tenants - List all tenants (super admin) - PUT /api/v1/admin/tenants/<tenant_id> - Update/rename tenant - POST /api/v1/admin/tenants/<tenant_id>/deactivate - Deactivate tenant - POST /api/v1/admin/tenants/<tenant_id>/reactivate - Reactivate tenant - DELETE /api/v1/admin/tenants/<tenant_id> - Delete tenant (dangerous) Currently tenants are created manually via: 1. Adding entry to tenant_client_keys table 2. Setting up Vault secrets for the tenant Needs: - Super admin permission check - Vault integration for tenant secrets - Database schema for tenants table (if needed) - Audit logging for all operations