fix: 为图片生成 API 添加超时与 5xx 重试机制

- ImageGenConfig 新增 timeout/max_retries/retry_delay 配置项（默认 120s/2次/5s）
- 替换 http.DefaultClient 为带超时的 imageHTTPClient
- callImageAPI 对 5xx 错误自动重试，避免 504 等瞬时故障直接失败

backend/internal/config/config.go

@@ -46,12 +46,15 @@ type LLMConfig struct {
 
 // ImageGenConfig OpenAI 兼容文生图模型配置。
 type ImageGenConfig struct {
-	BaseURL string `mapstructure:"base_url"`
-	APIKey  string `mapstructure:"api_key"`
-	Model   string `mapstructure:"model"`
-	Width   int    `mapstructure:"width"`
-	Height  int    `mapstructure:"height"`
-	Quality string `mapstructure:"quality"`
+	BaseURL      string `mapstructure:"base_url"`
+	APIKey       string `mapstructure:"api_key"`
+	Model        string `mapstructure:"model"`
+	Width        int    `mapstructure:"width"`
+	Height       int    `mapstructure:"height"`
+	Quality      string `mapstructure:"quality"`
+	Timeout      int    `mapstructure:"timeout"`       // HTTP 请求超时秒数，默认 120
+	MaxRetries   int    `mapstructure:"max_retries"`   // 5xx 错误重试次数，默认 2
+	RetryDelay   int    `mapstructure:"retry_delay"`   // 重试间隔秒数，默认 5
 }
 
 // QiniuConfig 七牛云对象存储配置。
@@ -117,6 +120,9 @@ func setDefaults(v *viper.Viper) {
 	v.SetDefault("image_gen.width", 1024)
 	v.SetDefault("image_gen.height", 1024)
 	v.SetDefault("image_gen.quality", "low")
+	v.SetDefault("image_gen.timeout", 120)
+	v.SetDefault("image_gen.max_retries", 2)
+	v.SetDefault("image_gen.retry_delay", 5)
 	v.SetDefault("image_gen.num_images", 1)
 	v.SetDefault("image_gen.steps", 30)
 	v.SetDefault("image_gen.cfg_scale", 7.0)
@@ -148,6 +154,9 @@ func bindEnvVars(v *viper.Viper) {
 	v.BindEnv("image_gen.width", "GEN2D_IMAGE_WIDTH")
 	v.BindEnv("image_gen.height", "GEN2D_IMAGE_HEIGHT")
 	v.BindEnv("image_gen.quality", "GEN2D_IMAGE_QUALITY")
+	v.BindEnv("image_gen.timeout", "GEN2D_IMAGE_TIMEOUT")
+	v.BindEnv("image_gen.max_retries", "GEN2D_IMAGE_MAX_RETRIES")
+	v.BindEnv("image_gen.retry_delay", "GEN2D_IMAGE_RETRY_DELAY")
 	v.BindEnv("image_gen.num_images", "GEN2D_IMAGE_NUM_IMAGES")
 	v.BindEnv("image_gen.steps", "GEN2D_IMAGE_STEPS")
 	v.BindEnv("image_gen.cfg_scale", "GEN2D_IMAGE_CFG_SCALE")