feat: wire real LLM runner via third-one or OpenAI-compatible API

internal/config/config.go

@@ -19,6 +19,8 @@ type Config struct {
 	ModelKey        string
 	ImageModelKey   string
 	ThirdOneBin     string
+	LLMAPIKey       string
+	LLMEndpoint     string
 	GoogleClientID  string
 	JWTSecret       string
 }
@@ -32,11 +34,17 @@ func LoadFromEnv() Config {
 		ModelKey:        envOrDefault("TUTOR_MODEL_KEY", defaultModelKey),
 		ImageModelKey:   envOrDefault("TUTOR_IMAGE_MODEL_KEY", defaultImageModelKey),
 		ThirdOneBin:     envOrDefault("THIRDONE_BIN", defaultThirdOneBin),
+		LLMAPIKey:       envOrDefault("TUTOR_LLM_API_KEY", ""),
+		LLMEndpoint:     envOrDefault("TUTOR_LLM_ENDPOINT", ""),
 		GoogleClientID:  envOrDefault("GOOGLE_CLIENT_ID", ""),
 		JWTSecret:       envOrDefault("JWT_SECRET", ""),
 	}
 }
 
+func (c Config) HasLLM() bool {
+	return c.LLMEndpoint != ""
+}
+
 func envOrDefault(key string, fallback string) string {
 	value := os.Getenv(key)
 	if value == "" {