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>

backend/app/schemas/task.py

@@ -54,11 +54,15 @@ class PreprocessingContrastEnum(str, Enum):
 
     - NONE: No contrast enhancement
     - HISTOGRAM: Standard histogram equalization
-    - CLAHE: Contrast Limited Adaptive Histogram Equalization (recommended)
+    - CLAHE: Contrast Limited Adaptive Histogram Equalization (recommended for most cases)
+    - DOCUMENT: Background normalization + CLAHE (recommended for scanned documents)
+      Removes uneven illumination before enhancement. Best for scans with
+      yellowed paper, shadow, or scanner lighting issues.
     """
     NONE = "none"
     HISTOGRAM = "histogram"
     CLAHE = "clahe"
+    DOCUMENT = "document"
 
 
 class PreprocessingConfig(BaseModel):