feat: upgrade PP-StructureV3 models to latest versions

- Layout: PP-DocLayout-S → PP-DocLayout_plus-L (83.2% mAP)
- Table: Single model → Dual SLANeXt (wired/wireless)
- Formula: PP-FormulaNet_plus-L for enhanced recognition
- Add preprocessing flags support (orientation, unwarping)
- Update frontend i18n descriptions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

openspec/changes/upgrade-ppstructure-models/tasks.md

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+# Tasks: Upgrade PP-StructureV3 Models
+
+## 1. Backend Configuration Changes
+
+- [x] 1.1 Update `backend/app/core/config.py` - Enable preprocessing flags
+  - Set `use_doc_orientation_classify` default to True
+  - Set `use_doc_unwarping` default to True
+  - Set `use_textline_orientation` default to True
+  - Add `table_structure_model_name` configuration
+  - Add `formula_recognition_model_name` configuration
+
+- [x] 1.2 Update `backend/app/services/ocr_service.py` - Model mapping changes
+  - Update `LAYOUT_MODEL_MAPPING`:
+    - Change `"chinese"` from `"PP-DocLayout-S"` to `"PP-DocLayout_plus-L"`
+    - Keep `"default"` as PubLayNet
+    - Keep `"cdla"` as is
+  - Update `_ensure_structure_engine()`:
+    - Pass preprocessing flags to PPStructureV3
+    - Configure SLANeXt models for table recognition
+    - Configure PP-FormulaNet_plus-L for formula recognition
+
+- [x] 1.3 Update PPStructureV3 initialization kwargs
+  - Add `table_structure_model_name="SLANeXt_wired"` (or configure dual model)
+  - Add `formula_recognition_model_name="PP-FormulaNet_plus-L"`
+  - Verify preprocessing flags are passed correctly
+
+## 2. Schema Updates
+
+- [x] 2.1 Update `backend/app/schemas/task.py` - LayoutModelEnum
+  - Rename or update `CHINESE` description to reflect PP-DocLayout_plus-L
+  - Update docstrings to reflect new model capabilities
+
+## 3. Frontend Updates
+
+- [x] 3.1 Update `frontend/src/components/LayoutModelSelector.tsx`
+  - Update Chinese option description to mention PP-DocLayout_plus-L
+  - Update accuracy information displayed to users
+
+- [x] 3.2 Update `frontend/src/i18n/locales/zh-TW.json`
+  - Update `layoutModel.chinese.description` to reflect new model
+  - Update any accuracy percentages in descriptions
+
+## 4. Testing
+
+- [x] 4.1 Create unit tests for new model configuration
+  - Test preprocessing flags are correctly passed
+  - Test model mapping resolves correctly
+  - Test engine initialization with new models
+
+- [ ] 4.2 Integration testing with real documents
+  - Test rotated document handling (preprocessing)
+  - Test complex Chinese document layout detection
+  - Test table structure recognition accuracy
+  - Test formula recognition with Chinese formulas
+
+- [x] 4.3 Update existing tests
+  - Update `backend/tests/services/test_layout_model.py` for new mapping
+  - Update `backend/tests/api/test_layout_model_api.py` if needed
+
+## 5. Documentation
+
+- [x] 5.1 Create model cleanup documentation
+  - Document `~/.paddlex/official_models/` cache location
+  - List models that can be safely deleted after upgrade
+  - Provide cleanup script/commands
+  - See: [MODEL_CLEANUP.md](./MODEL_CLEANUP.md)
+
+- [x] 5.2 Update API documentation
+  - Document preprocessing feature behavior
+  - Update layout model descriptions
+
+## 6. Verification & Deployment
+
+- [ ] 6.1 Verify new models download correctly on first use
+- [ ] 6.2 Measure memory/GPU usage with new models
+- [ ] 6.3 Compare processing speed before/after upgrade
+- [ ] 6.4 Verify existing functionality not broken