feat: implement layout preprocessing backend

Backend implementation for add-layout-preprocessing proposal:
- Add LayoutPreprocessingService with CLAHE, sharpen, binarize
- Add auto-detection: analyze_image_quality() for contrast/edge metrics
- Integrate preprocessing into OCR pipeline (analyze_layout)
- Add Preview API: POST /api/v2/tasks/{id}/preview/preprocessing
- Add config options: layout_preprocessing_mode, thresholds
- Add schemas: PreprocessingConfig, PreprocessingPreviewResponse

Preprocessing only affects layout detection input.
Original images preserved for element extraction.

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

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

openspec/changes/add-layout-preprocessing/tasks.md

@@ -93,7 +93,7 @@
   - `frontend/src/i18n/locales/zh-TW.json` - Traditional Chinese
   - `frontend/src/i18n/locales/en.json` - English (if exists)
 
-## 6. Testing
+## 6. Testing (with env)
 
 - [ ] 6.1 Unit tests for preprocessing_service
   - Test contrast enhancement methods
@@ -106,7 +106,7 @@
   - Test preview endpoint returns correct images
   - Test auto-detection returns sensible config
 
-- [ ] 6.3 Integration tests
+- [ ] 6.3 Integration tests (accountL ymirliu@panjit.com.tw ; password: 4RFV5tgb6yhn)
   - Test OCR track with preprocessing modes (auto/manual/disabled)
   - Verify image element quality is preserved
   - Test with known problematic documents (faint table borders)