feat: implement layout preprocessing backend

Backend implementation for add-layout-preprocessing proposal:
- Add LayoutPreprocessingService with CLAHE, sharpen, binarize
- Add auto-detection: analyze_image_quality() for contrast/edge metrics
- Integrate preprocessing into OCR pipeline (analyze_layout)
- Add Preview API: POST /api/v2/tasks/{id}/preview/preprocessing
- Add config options: layout_preprocessing_mode, thresholds
- Add schemas: PreprocessingConfig, PreprocessingPreviewResponse

Preprocessing only affects layout detection input.
Original images preserved for element extraction.

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

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

backend/app/schemas/task.py

@@ -37,6 +37,79 @@ class LayoutModelEnum(str, Enum):
     CDLA = "cdla"         # CDLA model - Alternative for Chinese layout
 
 
+class PreprocessingModeEnum(str, Enum):
+    """Preprocessing mode for layout detection enhancement.
+
+    - AUTO: Analyze image quality and automatically apply optimal preprocessing
+    - MANUAL: Use user-specified preprocessing configuration
+    - DISABLED: Skip preprocessing entirely
+    """
+    AUTO = "auto"         # Analyze and apply automatically (default)
+    MANUAL = "manual"     # Use specified configuration
+    DISABLED = "disabled" # Skip preprocessing
+
+
+class PreprocessingContrastEnum(str, Enum):
+    """Contrast enhancement method for preprocessing.
+
+    - NONE: No contrast enhancement
+    - HISTOGRAM: Standard histogram equalization
+    - CLAHE: Contrast Limited Adaptive Histogram Equalization (recommended)
+    """
+    NONE = "none"
+    HISTOGRAM = "histogram"
+    CLAHE = "clahe"
+
+
+class PreprocessingConfig(BaseModel):
+    """Preprocessing configuration for layout detection enhancement.
+
+    Used to configure image preprocessing before PP-Structure layout detection.
+    Preprocessing helps detect tables with faint lines or low contrast borders.
+    Original image is preserved for element extraction.
+    """
+    contrast: PreprocessingContrastEnum = Field(
+        default=PreprocessingContrastEnum.CLAHE,
+        description="Contrast enhancement method"
+    )
+    sharpen: bool = Field(
+        default=True,
+        description="Enable sharpening for faint lines"
+    )
+    binarize: bool = Field(
+        default=False,
+        description="Enable binarization (aggressive, for very low contrast)"
+    )
+
+
+class ImageQualityMetrics(BaseModel):
+    """Image quality metrics from auto-analysis."""
+    contrast: float = Field(..., description="Contrast level (std dev of grayscale)")
+    edge_strength: float = Field(..., description="Edge strength (Sobel gradient mean)")
+
+
+class PreprocessingPreviewRequest(BaseModel):
+    """Request for preprocessing preview."""
+    page: int = Field(default=1, ge=1, description="Page number to preview")
+    mode: PreprocessingModeEnum = Field(
+        default=PreprocessingModeEnum.AUTO,
+        description="Preprocessing mode"
+    )
+    config: Optional[PreprocessingConfig] = Field(
+        None,
+        description="Manual configuration (only used when mode='manual')"
+    )
+
+
+class PreprocessingPreviewResponse(BaseModel):
+    """Response for preprocessing preview."""
+    original_url: str = Field(..., description="URL to original image")
+    preprocessed_url: str = Field(..., description="URL to preprocessed image")
+    quality_metrics: ImageQualityMetrics = Field(..., description="Image quality analysis")
+    auto_config: PreprocessingConfig = Field(..., description="Auto-detected configuration")
+    mode_used: PreprocessingModeEnum = Field(..., description="Mode that was applied")
+
+
 class TaskCreate(BaseModel):
     """Task creation request"""
     filename: Optional[str] = Field(None, description="Original filename")
@@ -195,6 +268,16 @@ class ProcessingOptions(BaseModel):
         description="Layout detection model: 'chinese' (recommended for Chinese docs), 'default' (English docs), 'cdla' (Chinese layout)"
     )
 
+    # Layout preprocessing (OCR track only)
+    preprocessing_mode: PreprocessingModeEnum = Field(
+        default=PreprocessingModeEnum.AUTO,
+        description="Preprocessing mode: 'auto' (analyze and apply), 'manual' (use config), 'disabled'"
+    )
+    preprocessing_config: Optional[PreprocessingConfig] = Field(
+        None,
+        description="Manual preprocessing config (only used when preprocessing_mode='manual')"
+    )
+
 
 class AnalyzeRequest(BaseModel):
     """Document analysis request"""