feat(image): 支持真 16:9(1536x864)+ 人物/商品三视图/场景默认 16:9

- gpt-image-2 实测支持 16:9(宽高需被 16 整除):真 16:9 = 1536x864(长边封顶 1536);
  原来「16:9」误给成 1536x1024(实为 3:2)。修正 _RATIO_TO_SIZE 与 _ratio_to_image_size
- admin「提示词」页比例按钮新增「宽 16:9」
- 迁移 0015/0016:scene + product_triview + person_triview 默认比例设为 16:9;
  分镜图(storyboard_frame)/立绘/视频保持竖屏不动

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/frontend/src/routes/admin/admin-prompts.tsx

@@ -10,7 +10,8 @@ const RATIOS = [
   { key: "", label: "默认" },
   { key: "1:1", label: "1:1 方" },
   { key: "portrait", label: "竖 2:3" },
-  { key: "landscape", label: "横 3:2" }
+  { key: "landscape", label: "横 3:2" },
+  { key: "16:9", label: "宽 16:9" }
 ];
 
 export function AdminPromptsPage({ notify }: { notify: Notify }) {