proposal: add hybrid control mode with auto-detection and preview

Updates add-layout-preprocessing proposal:
- Auto mode: analyze image quality, auto-select parameters
- Manual mode: user override with specific settings
- Preview API: compare original vs preprocessed before processing
- Frontend UI: mode selection, manual controls, preview button

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

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

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

@@ -18,9 +18,19 @@ The root cause is that layout detection happens **before** table structure recog
   - 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
+- **Hybrid control mode** (Auto + Manual)
+  - **Auto mode (default)**: Analyze image quality and auto-select parameters
+    - Calculate contrast level (standard deviation)
+    - Detect edge clarity for faint lines
+    - Apply appropriate preprocessing based on analysis
+  - **Manual mode**: User can override with specific settings
+    - Contrast: none / histogram / clahe
+    - Sharpen: on/off
+    - Binarize: on/off
+
+- **Frontend preview API**
+  - Preview endpoint to show original vs preprocessed comparison
+  - Users can verify settings before processing
 
 ## Impact
 
@@ -31,6 +41,8 @@ The root cause is that layout detection happens **before** table structure recog
 - `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)
+- `backend/app/api/v2/endpoints/preview.py` - New preview API endpoint
+- `frontend/src/components/PreprocessingSettings.tsx` - New UI component
 
 ### Track Impact Analysis