chore: filter /v1/models to allowed providers

Restricted model listing to OpenAI, Google (Gemini), DeepSeek, and xAI (Grok) to match available access.

internal/server/server.go

@@ -150,13 +150,23 @@ func (s *Server) handleListModels(c *gin.Context) {
 	}
 
 	var data []OpenAIModel
+	allowedProviders := map[string]bool{
+		"openai":   true,
+		"google":   true, // Models from models.dev use 'google' ID for Gemini
+		"deepseek": true,
+		"xai":      true, // Models from models.dev use 'xai' ID for Grok
+	}
+
 	if s.registry != nil {
 		for pID, pInfo := range s.registry.Providers {
+			if !allowedProviders[pID] {
+				continue
+			}
 			for mID := range pInfo.Models {
 				data = append(data, OpenAIModel{
 					ID:      mID,
 					Object:  "model",
-					Created: 1700000000, // Static placeholder
+					Created: 1700000000,
 					OwnedBy: pID,
 				})
 			}