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/proposal.md

@@ -0,0 +1,62 @@
+# Change: Add Image Preprocessing for Layout Detection
+
+## Why
+
+PP-StructureV3's layout detection (PP-DocLayout_plus-L) sometimes fails to detect tables with faint lines, low contrast borders, or poor scan quality. This results in missing table elements in the output, even when the table structure recognition models (SLANeXt) are correctly configured.
+
+The root cause is that layout detection happens **before** table structure recognition - if a region isn't identified as a "table" in the layout detection stage, the table recognition models never get invoked.
+
+## What Changes
+
+- **Add image preprocessing module** for layout detection input
+  - Contrast enhancement (histogram equalization, CLAHE)
+  - Optional binarization (adaptive thresholding)
+  - Sharpening for faint lines
+
+- **Preserve original images for extraction**
+  - Preprocessing ONLY affects layout detection input
+  - Image element extraction continues to use original (preserves quality)
+  - Raw OCR continues to use original image
+
+- **Configurable preprocessing options**
+  - Enable/disable preprocessing per track
+  - Adjustable preprocessing intensity
+
+## Impact
+
+### Affected Specs
+- `ocr-processing` - New preprocessing configuration requirements
+
+### Affected Code
+- `backend/app/services/ocr_service.py` - Add preprocessing before PP-Structure
+- `backend/app/core/config.py` - New preprocessing configuration options
+- `backend/app/services/preprocessing_service.py` - New service (to be created)
+
+### Track Impact Analysis
+
+| Track | Impact | Reason |
+|-------|--------|--------|
+| OCR | Improved layout detection | Preprocessing enhances PP-Structure input |
+| Hybrid | Potentially improved | Uses PP-Structure for layout |
+| Direct | No impact | Does not use PP-Structure |
+| Raw OCR | No impact | Continues using original image |
+
+### Quality Impact
+
+| Component | Impact | Reason |
+|-----------|--------|--------|
+| Table detection | Improved | Enhanced contrast reveals faint borders |
+| Image extraction | No change | Uses original image for quality |
+| Text recognition | No change | Raw OCR uses original image |
+| Reading order | Improved | Better element detection → better ordering |
+
+## Risks
+
+1. **Performance overhead**: Preprocessing adds compute time per page
+   - Mitigation: Make preprocessing optional, cache preprocessed images
+
+2. **Over-processing**: Strong enhancement may introduce artifacts
+   - Mitigation: Configurable intensity levels, default to moderate enhancement
+
+3. **Memory usage**: Keeping both original and preprocessed images
+   - Mitigation: Preprocessed image is temporary, discarded after layout detection