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/router/router.go

@@ -36,6 +36,15 @@ func New(groups []db.ModelGroup, classify ClassifierFunc) *Router {
 	return r
 }
 
+// Groups returns all registered model group IDs.
+func (r *Router) Groups() []string {
+	ids := make([]string, 0, len(r.groups))
+	for id := range r.groups {
+		ids = append(ids, id)
+	}
+	return ids
+}
+
 // IsGroup returns true if the model name is a group ID.
 func (r *Router) IsGroup(modelID string) bool {
 	_, ok := r.groups[modelID]