feat: expose model groups in /v1/models endpoint

Add Groups() method to Router so handleListModels can append model
group IDs (e.g. 'deepseek-auto', 'openai-auto') to the model list,
marked with owned_by: 'gophergate'. This lets clients discover and
use groups via the standard OpenAI /v1/models endpoint.

internal/server/server.go

@@ -467,6 +467,20 @@ func (s *Server) handleListModels(c *gin.Context) {
 		}
 	}
 
+	// Add model groups so clients can discover them
+	if s.modelRouter != nil {
+		for _, gid := range s.modelRouter.Groups() {
+			if _, exists := modelMap[gid]; !exists {
+				modelMap[gid] = OpenAIModel{
+					ID:      gid,
+					Object:  "model",
+					Created: 1700000000,
+					OwnedBy: "gophergate",
+				}
+			}
+		}
+	}
+
 	var data []OpenAIModel
 	for _, m := range modelMap {
 		data = append(data, m)