动作视频背景色多色自动决策并接入阿里云抠帧

- 动画生成请求新增 screenColor，继承生图入口的 auto 决策行为，
  提示词从写死绿幕改为纯色背景条款（LLM 决策 11 色）
- 后端抽帧抠图改为逐帧阿里云优先（并发 3 保序），单帧失败降级
  本地键色算法（使用决策出的背景色）
- 清理 legacy 纯绿幕常量与提示词

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/components/image-editor/ImageCanvasGenerationSubmissionModel.test.ts

@@ -726,6 +726,7 @@ describe('ImageCanvasGenerationSubmissionModel', () => {
         sourceWidth: 960,
         sourceHeight: 1280,
         promptText: '循环奔跑动作',
+        screenColor: 'auto',
         resolution: '720p',
         ratio: 'same',
         frameCount: 48,

src/components/image-editor/ImageCanvasGenerationSubmissionModel.ts

@@ -649,6 +649,8 @@ export function buildCharacterAnimationSubmissionPlan({
       sourceWidth: sourceLayer.originalWidth,
       sourceHeight: sourceLayer.originalHeight,
       promptText,
+      // 与生图入口一致：默认 auto，由后端做背景色自动决策。
+      screenColor: DEFAULT_EDITOR_GENERATION_BACKGROUND_COLOR,
       resolution: panel.resolution,
       ratio: panel.ratio,
       frameCount: panel.frameCount,

src/services/image-editor/editorProjectClient.ts

@@ -328,6 +328,8 @@ export type EditorCharacterAnimationGenerationInput = {
   sourceWidth: number;
   sourceHeight: number;
   promptText: string;
+  /** 纯色抠像背景色（hex 或 'auto'），继承生图的背景色选项行为。 */
+  screenColor?: string;
   resolution: EditorCharacterAnimationResolution;
   ratio: EditorCharacterAnimationRatio;
   frameCount: EditorCharacterAnimationFrameCount;