feat: comprehensive SSO/OIDC upgrade with enterprise features

Multi-provider SSO support:
   - Database-backed SSO provider management with admin UI
   - Support for OpenID Connect, Google OAuth2, GitHub, and SAML 2.0
   - Flipper feature flag (db_sso_providers) for dynamic provider loading
   - ProviderLoader service for YAML or database configuration

   Admin functionality:
   - Admin::SsoProvidersController for CRUD operations
   - Admin::UsersController for super_admin role management
   - Pundit policies for authorization
   - Test connection endpoint for validating provider config

   User provisioning improvements:
   - JIT (just-in-time) account creation with configurable default role
   - Changed default JIT role from admin to member (security)
   - User attribute sync on each SSO login
   - Group/role mapping from IdP claims

   SSO identity management:
   - Settings::SsoIdentitiesController for users to manage connected accounts
   - Issuer validation for OIDC identities
   - Unlink protection when no password set

   Audit logging:
   - SsoAuditLog model tracking login, logout, link, unlink, JIT creation
   - Captures IP address, user agent, and metadata

   Advanced OIDC features:
   - Custom scopes per provider
   - Configurable prompt parameter (login, consent, select_account, none)
   - RP-initiated logout (federated logout to IdP)
   - id_token storage for logout

   SAML 2.0 support:
   - omniauth-saml gem integration
   - IdP metadata URL or manual configuration
   - Certificate and fingerprint validation
   - NameID format configuration

app/controllers/admin/users_controller.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module Admin
+  class UsersController < Admin::BaseController
+    before_action :set_user, only: %i[update]
+
+    def index
+      authorize User
+      @users = policy_scope(User).order(:email)
+    end
+
+    def update
+      authorize @user
+
+      if @user.update(user_params)
+        Rails.logger.info(
+          "[Admin::Users] Role changed - " \
+          "by_user_id=#{Current.user.id} " \
+          "target_user_id=#{@user.id} " \
+          "new_role=#{@user.role}"
+        )
+        redirect_to admin_users_path, notice: t(".success")
+      else
+        redirect_to admin_users_path, alert: t(".failure")
+      end
+    end
+
+    private
+
+      def set_user
+        @user = User.find(params[:id])
+      end
+
+      def user_params
+        params.require(:user).permit(:role)
+      end
+  end
+end