修复序列帧去背景队列与幂等发布

序列帧去背景改用本地绿幕算法并在 durable 父任务内并行处理。

将去背景操作加入生成结果白名单，并按 operation_id 隔离帧对象路径。

去背景结果发布为新的序列资源、素材和画布图层，同时保留原始序列。

同步前端生成占位、相关测试及队列与资源契约文档。

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

@@ -14,6 +14,7 @@ import {
   closeGenerateComposerDialog,
   createAudioRedrawGenerationDialogDraft,
   createBackgroundMusicGenerationDialogDraft,
+  createCharacterAnimationBackgroundRemovalDialogDraft,
   createCharacterAnimationGenerationDialogDraft,
   createCharacterAnimationPanelDraft,
   createCharacterGenerationDialogDraft,
@@ -559,6 +560,33 @@ describe('ImageCanvasGenerationDialogModel', () => {
     });
   });
 
+  it('creates a separate character animation placement for background removal', () => {
+    expect(
+      createCharacterAnimationBackgroundRemovalDialogDraft({
+        sourceLayer: createLayer({
+          id: 'layer-animation',
+          assetKind: 'character-animation',
+        }),
+        assetLabel: '源动画 去背景',
+      }),
+    ).toMatchObject({
+      mode: 'character-animation',
+      prompt: '去除背景',
+      assetLabel: '源动画 去背景',
+      status: 'generating',
+      composerOpen: false,
+      sourceLayerId: 'layer-animation',
+      placeholder: {
+        x: 472,
+        y: 140,
+        width: 320,
+        height: 240,
+        originalWidth: 1024,
+        originalHeight: 768,
+      },
+    });
+  });
+
   it('restores remodel prompts from user-visible generation input snapshots only', () => {
     const cases: Array<{
       fields: NonNullable<CanvasLayer['generationInputs']>['fields'];