feat: enhance layout preprocessing and unify image scaling proposal

Backend changes:
- Add image scaling configuration for PP-Structure processing
- Enhance layout preprocessing service with scaling support
- Update OCR service with improved memory management
- Add PP-Structure enhanced processing improvements

Frontend changes:
- Update preprocessing settings UI
- Fix processing page layout and state management
- Update API types for new parameters

Proposals:
- Archive add-layout-preprocessing proposal (completed)
- Add unify-image-scaling proposal for consistent coordinate handling

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

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

openspec/changes/unify-image-scaling/tasks.md

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+# Tasks: Unify Image Scaling Strategy
+
+## 1. Configuration
+
+- [x] 1.1 Add min_dimension setting to `backend/app/core/config.py`
+  - `layout_image_scaling_min_dimension: int = 1200`
+  - Description: "Min dimension (pixels) before upscaling. Images smaller than this will be scaled up."
+
+## 2. Bidirectional Scaling Logic
+
+- [x] 2.1 Update `scale_for_layout_detection()` in `layout_preprocessing_service.py`
+  - Add upscaling condition: `max_dim < min_dimension`
+  - Use `cv2.INTER_CUBIC` for upscaling (better quality than INTER_LINEAR)
+  - Update docstring to reflect bidirectional behavior
+
+- [x] 2.2 Update scaling decision logic
+  ```python
+  # Current: only downscale
+  should_scale = max_dim > max_dimension
+
+  # New: bidirectional
+  should_downscale = max_dim > max_dimension
+  should_upscale = max_dim < min_dimension
+  should_scale = should_downscale or should_upscale
+  ```
+
+- [x] 2.3 Update logging to indicate scale direction
+  - "Scaled DOWN for layout detection: 2480x3508 -> 1131x1600"
+  - "Scaled UP for layout detection: 800x600 -> 1600x1200"
+
+## 3. PDF DPI Handling (Optional Optimization)
+
+- [x] 3.1 Evaluate current PDF conversion impact
+  - Decision: Keep 300 DPI, let bidirectional scaling handle it
+  - Reason: Raw OCR benefits from high resolution, scaling handles PP-Structure needs
+
+- [x] 3.2 Option A: Keep 300 DPI, let scaling handle it ✓
+  - Simplest approach, no change needed
+  - Raw OCR benefits from high resolution
+
+- [ ] ~~3.3 Option B: Add configurable PDF DPI~~ (Not needed)
+
+## 4. Testing
+
+- [x] 4.1 Test upscaling with small images
+  - Small image (800x600): Scaled UP → 1600x1200, scale_factor=0.500
+  - Very small (400x300): Scaled UP → 1600x1200, scale_factor=0.250
+
+- [x] 4.2 Test no scaling for optimal range
+  - Optimal image (1500x1000): was_scaled=False, scale_factor=1.000
+
+- [x] 4.3 Test downscaling (existing behavior)
+  - Large image (2480x3508): Scaled DOWN → 1131x1600, scale_factor=2.192
+
+- [ ] 4.4 Test PDF workflow (manual test recommended)
+  - PDF page should be detected correctly
+  - Scaling should apply after PDF conversion
+
+## 5. Documentation
+
+- [x] 5.1 Update config.py Field descriptions
+  - Explained bidirectional scaling in enabled field description
+  - Updated max/min/target descriptions
+
+- [x] 5.2 Add logging for scaling decisions
+  - Logs direction (UP/DOWN), original size, target size, scale_factor
+
+---
+
+## Implementation Summary
+
+**Files Modified:**
+- `backend/app/core/config.py` - Added `layout_image_scaling_min_dimension` setting
+- `backend/app/services/layout_preprocessing_service.py` - Updated bidirectional scaling logic
+
+**Test Results (2025-11-27):**
+| Test Case | Original | Result | scale_factor |
+|-----------|----------|--------|--------------|
+| Small (800×600) | max=800 < 1200 | UP → 1600×1200 | 0.500 |
+| Optimal (1500×1000) | 1200 ≤ 1500 ≤ 2000 | No scaling | 1.000 |
+| Large (2480×3508) | max=3508 > 2000 | DOWN → 1131×1600 | 2.192 |
+| Very small (400×300) | max=400 < 1200 | UP → 1600×1200 | 0.250 |
+
+---
+
+## Implementation Notes
+
+### Scaling Decision Matrix
+
+| Image Size | Action | Scale Factor | Interpolation |
+|------------|--------|--------------|---------------|
+| < 1200px | Scale UP | target/max_dim | INTER_CUBIC |
+| 1200-2000px | No scaling | 1.0 | N/A |
+| > 2000px | Scale DOWN | target/max_dim | INTER_AREA |
+
+### Example Scenarios
+
+1. **Small scan (800×600)**
+   - max_dim = 800 < 1200 → Scale UP
+   - target = 1600, scale = 1600/800 = 2.0
+   - Result: 1600×1200
+   - scale_factor (for bbox restore) = 0.5
+
+2. **Optimal image (1400×1000)**
+   - max_dim = 1400, 1200 <= 1400 <= 2000 → No scaling
+   - Result: unchanged
+   - scale_factor = 1.0
+
+3. **High-res scan (2480×3508)**
+   - max_dim = 3508 > 2000 → Scale DOWN
+   - target = 1600, scale = 1600/3508 = 0.456
+   - Result: 1131×1600
+   - scale_factor (for bbox restore) = 2.19