feat: complete core data models

Add Template, ExportOptions, BatchMode, BatchOptions, ComposeResult,
BatchResult to models.py. Keep existing ImageAsset and TransformState,
adding center() helper to TransformState.

Template uses ratio-based fields with to_transform_state() to convert
to pixel coordinates for any garment size.
BatchMode enum covers all four batch modes from PRD.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tasks.md

@@ -133,21 +133,21 @@
 
 任务：
 
-- [ ] 先读取 `src/core/models.py` 现有内容
-- [ ] 完善 `src/core/models.py`
-- [ ] 定义 `ImageAsset`
-- [ ] 定义 `TransformState`
-- [ ] 定义 `Template`
-- [ ] 定义 `ExportOptions`
-- [ ] 定义 `BatchOptions`
-- [ ] 定义 `ComposeResult`
-- [ ] 定义 `BatchResult`
+- [x] 先读取 `src/core/models.py` 现有内容
+- [x] 完善 `src/core/models.py`
+- [x] 定义 `ImageAsset`
+- [x] 定义 `TransformState`
+- [x] 定义 `Template`
+- [x] 定义 `ExportOptions`
+- [x] 定义 `BatchOptions`
+- [x] 定义 `ComposeResult`
+- [x] 定义 `BatchResult`
 
 验收：
 
-- [ ] `models.py` 不依赖 PySide6 UI 控件
-- [ ] 变换状态包含 `x/y/width/height/rotation/keep_aspect_ratio`
-- [ ] 模型可以被 core、services 和 UI 层复用
+- [x] `models.py` 不依赖 PySide6 UI 控件
+- [x] 变换状态包含 `x/y/width/height/rotation/keep_aspect_ratio`
+- [x] 模型可以被 core、services 和 UI 层复用
 
 ## 3. 文件扫描与素材导入