proposal: add-layout-preprocessing for improved table detection

Problem: PP-Structure misses tables with faint lines/borders
Solution: Preprocess images (contrast, sharpen) for layout detection
- Preprocessed image only used for layout detection
- Original image preserved for element extraction (quality)

Includes: proposal.md, design.md, tasks.md, spec delta

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

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

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

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+# Tasks: Add Image Preprocessing for Layout Detection
+
+## 1. Configuration
+
+- [ ] 1.1 Add preprocessing configuration to `backend/app/core/config.py`
+  - `layout_preprocessing_enabled: bool = True` - Enable/disable preprocessing
+  - `layout_preprocessing_contrast: str = "clahe"` - Options: none, histogram, clahe
+  - `layout_preprocessing_sharpen: bool = True` - Enable sharpening for faint lines
+  - `layout_preprocessing_binarize: bool = False` - Optional binarization (aggressive)
+
+## 2. Preprocessing Service
+
+- [ ] 2.1 Create `backend/app/services/preprocessing_service.py`
+  - Image loading utility (supports PIL, OpenCV)
+  - Contrast enhancement methods (histogram equalization, CLAHE)
+  - Sharpening filter for line enhancement
+  - Optional adaptive binarization
+  - Return preprocessed image as numpy array or PIL Image
+
+- [ ] 2.2 Implement `enhance_for_layout_detection()` function
+  - Input: Original image path or PIL Image
+  - Output: Preprocessed image (same format as input)
+  - Steps: contrast → sharpen → (optional) binarize
+
+## 3. Integration with OCR Service
+
+- [ ] 3.1 Update `backend/app/services/ocr_service.py`
+  - Import preprocessing service
+  - Before `_run_ppstructure()`, preprocess image if enabled
+  - Pass preprocessed image to PP-Structure for layout detection
+  - Keep original image reference for image extraction
+
+- [ ] 3.2 Ensure image element extraction uses original
+  - Verify `saved_path` and `img_path` in elements reference original
+  - Bbox coordinates from preprocessed detection applied to original crop
+
+## 4. Testing
+
+- [ ] 4.1 Unit tests for preprocessing_service
+  - Test contrast enhancement methods
+  - Test sharpening filter
+  - Test binarization
+  - Test with various image formats (PNG, JPEG)
+
+- [ ] 4.2 Integration tests
+  - Test OCR track with preprocessing enabled/disabled
+  - Verify image element quality is preserved
+  - Test with known problematic documents (faint table borders)
+
+## 5. Documentation
+
+- [ ] 5.1 Update API documentation
+  - Document new configuration options
+  - Explain preprocessing behavior