feat(auth): add superadmin support for OAuth clients

* Introduce Superadmin field in OAuthClient configuration
* Implement IsSuperadmin method in OAuthRegistry
* Update identityAdmin to check superadmin status via OAuthRegistry
* Add tests for OAuthRegistry superadmin functionality

internal/auth/oauth_registry.go

@@ -31,3 +31,18 @@ func (o *OAuthRegistry) Lookup(clientID string, clientSecret string) (string, bo
 	}
 	return "", false
 }
+
+// IsSuperadmin reports whether keyID (as returned by Lookup) belongs to an
+// OAuth client configured with superadmin: true.
+func (o *OAuthRegistry) IsSuperadmin(keyID string) bool {
+	for _, client := range o.clients {
+		id := client.ID
+		if id == "" {
+			id = client.ClientID
+		}
+		if id == keyID {
+			return client.Superadmin
+		}
+	}
+	return false
+}

internal/auth/oauth_registry_test.go

@@ -32,6 +32,26 @@ func TestNewOAuthRegistryAndLookup(t *testing.T) {
 	}
 }
 
+func TestOAuthRegistryIsSuperadmin(t *testing.T) {
+	registry, err := NewOAuthRegistry([]config.OAuthClient{
+		{ID: "oauth-admin", ClientID: "admin-id", ClientSecret: "admin-secret", Superadmin: true},
+		{ID: "oauth-client", ClientID: "client-id", ClientSecret: "client-secret"},
+	})
+	if err != nil {
+		t.Fatalf("NewOAuthRegistry() error = %v", err)
+	}
+
+	if !registry.IsSuperadmin("oauth-admin") {
+		t.Fatal("IsSuperadmin(oauth-admin) = false, want true")
+	}
+	if registry.IsSuperadmin("oauth-client") {
+		t.Fatal("IsSuperadmin(oauth-client) = true, want false")
+	}
+	if registry.IsSuperadmin("unknown") {
+		t.Fatal("IsSuperadmin(unknown) = true, want false")
+	}
+}
+
 func TestMiddlewareAllowsOAuthBasicAuthAndSetsContext(t *testing.T) {
 	oauthRegistry, err := NewOAuthRegistry([]config.OAuthClient{{
 		ID:           "oauth-client",