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/middleware/omniauth_error_handler.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+# Middleware to catch OmniAuth/OIDC errors and redirect gracefully
+# instead of showing ugly error pages
+class OmniauthErrorHandler
+  def initialize(app)
+    @app = app
+  end
+
+  def call(env)
+    @app.call(env)
+  rescue OpenIDConnect::Discovery::DiscoveryFailed => e
+    Rails.logger.error("[OmniAuth] OIDC Discovery failed: #{e.message}")
+    redirect_to_failure(env, "sso_provider_unavailable")
+  rescue OmniAuth::Error => e
+    Rails.logger.error("[OmniAuth] Authentication error: #{e.message}")
+    redirect_to_failure(env, "sso_failed")
+  end
+
+  private
+
+    def redirect_to_failure(env, message)
+      [
+        302,
+        { "Location" => "/auth/failure?message=#{message}", "Content-Type" => "text/html" },
+        [ "Redirecting..." ]
+      ]
+    end
+end