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>

frontend/src/components/PreprocessingSettings.tsx

@@ -30,7 +30,7 @@ export default function PreprocessingSettings({
 }: PreprocessingSettingsProps) {
   const { t } = useTranslation()
   const modes: PreprocessingMode[] = ['auto', 'manual', 'disabled']
-  const contrastOptions: PreprocessingContrast[] = ['none', 'histogram', 'clahe']
+  const contrastOptions: PreprocessingContrast[] = ['none', 'histogram', 'clahe', 'document']
 
   const getModeInfo = (m: PreprocessingMode) => ({
     label: t(`processing.preprocessing.mode.${m}`),