收紧编辑器内部处理元数据边界

普通用户与公开响应统一移除生成 provider、内部处理模型和抠图审计字段。
普通生成请求移除 segModel，由后端固定抠图策略并清理客户端伪造的保留字段。
手动去背景继承可信源模型，角色动作与去背景失败统一使用稳定用户文案。
后台原始审计继续保留真实 provider 与抠图模型，并补齐 Owner、Public、Admin 和紧凑结果测试。
同步更新前端响应契约、外部 OpenAPI、后端架构文档和项目决策记录。

src/services/image-editor/editorProjectClient.test.ts

@@ -837,7 +837,6 @@ describe('editorProjectClient', () => {
       kind: 'character',
       model: 'gpt-image-2',
       screenColor: '#FFD6C2',
-      segModel: 'anime-seg',
       style: 'pixelArt',
       aspectRatio: '2:3',
       imageSize: '1K',
@@ -862,7 +861,6 @@ describe('editorProjectClient', () => {
           kind: 'character',
           model: 'gpt-image-2',
           screenColor: '#FFD6C2',
-          segModel: 'anime-seg',
           style: 'pixelArt',
           aspectRatio: '2:3',
           imageSize: '1K',
@@ -1016,7 +1014,6 @@ describe('editorProjectClient', () => {
       iconDescriptions: ['返回按钮'],
       model: 'gpt-image-2',
       screenColor: '#E6D8FF',
-      segModel: 'anime-seg',
       style: 'none',
       aspectRatio: '1:1',
       imageSize: '2K',
@@ -1038,7 +1035,6 @@ describe('editorProjectClient', () => {
           iconDescriptions: ['返回按钮'],
           model: 'gpt-image-2',
           screenColor: '#E6D8FF',
-          segModel: 'anime-seg',
           style: 'none',
           aspectRatio: '1:1',
           imageSize: '2K',
@@ -1157,7 +1153,6 @@ describe('editorProjectClient', () => {
       sourceImageSrc: '/generated-images/editor/ui-design.png',
       model: 'gpt-image-2',
       screenColor: '#7FB3FF',
-      segModel: 'anime-seg',
       referenceImageSrcs: ['/generated-images/editor/ui-ref.png'],
       projectId: 'editor-project-1',
       assetFolderId: 'project',
@@ -1180,7 +1175,6 @@ describe('editorProjectClient', () => {
           sourceImageSrc: '/generated-images/editor/ui-design.png',
           model: 'gpt-image-2',
           screenColor: '#7FB3FF',
-          segModel: 'anime-seg',
           referenceImageSrcs: ['/generated-images/editor/ui-ref.png'],
           projectId: 'editor-project-1',
           generationInputs: {