fix(worker): 限制大图归一化峰值内存

6K 多参考图在 P24 下会并发进入高内存缩放并击穿 2 GiB Worker。新增独立可配置的图片归一化并发，生产默认 2，只约束解码、缩放和重编码阶段，不占用视频上游等待槽；缩放器改为内存稳定的近似双线性实现。\n\n验证：6K 到 6000x2400 转换、信号量串行化、Go 全量测试、gofmt、Kubernetes 客户端 dry-run。

apps/api/internal/config/config_test.go

@@ -270,6 +270,15 @@ func TestValidateProcessRoleAndDatabasePool(t *testing.T) {
 	if got := Load().MediaRequestConcurrency; got != 96 {
 		t.Fatalf("media request concurrency = %d, want 96", got)
 	}
+	cfg = Load()
+	cfg.MediaImageNormalizationConcurrency = 65
+	if err := cfg.Validate(); err == nil || !strings.Contains(err.Error(), "MEDIA_IMAGE_NORMALIZATION_CONCURRENCY") {
+		t.Fatalf("Validate() error = %v, want invalid image normalization concurrency", err)
+	}
+	t.Setenv("AI_GATEWAY_MEDIA_IMAGE_NORMALIZATION_CONCURRENCY", "3")
+	if got := Load().MediaImageNormalizationConcurrency; got != 3 {
+		t.Fatalf("image normalization concurrency = %d, want 3", got)
+	}
 }
 
 func TestValidateTaskHistorySettings(t *testing.T) {