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/specs/ocr-processing/spec.md

@@ -17,12 +17,6 @@ The system SHALL provide optional image preprocessing to enhance layout detectio
 - **THEN** the system SHALL crop from the ORIGINAL image, not the preprocessed version
 - **AND** the extracted image SHALL maintain original quality and colors
 
-#### Scenario: Preprocessing can be disabled
-- **GIVEN** `layout_preprocessing_enabled` is set to false in configuration
-- **WHEN** OCR track processing runs
-- **THEN** the system SHALL skip preprocessing
-- **AND** PP-Structure SHALL receive the original image directly
-
 #### Scenario: CLAHE contrast enhancement
 - **WHEN** `layout_preprocessing_contrast` is set to "clahe"
 - **THEN** the system SHALL apply Contrast Limited Adaptive Histogram Equalization
@@ -38,6 +32,61 @@ The system SHALL provide optional image preprocessing to enhance layout detectio
 - **THEN** the system SHALL apply adaptive thresholding
 - **AND** this SHALL be used only for documents with very poor contrast
 
+### Requirement: Preprocessing Hybrid Control Mode
+
+The system SHALL support three preprocessing modes: automatic, manual, and disabled, with automatic as the default.
+
+#### Scenario: Auto mode analyzes image quality
+- **GIVEN** preprocessing mode is set to "auto"
+- **WHEN** processing begins for a page
+- **THEN** the system SHALL analyze image quality metrics (contrast, edge strength)
+- **AND** automatically determine optimal preprocessing parameters
+- **AND** apply recommended settings without user intervention
+
+#### Scenario: Auto mode detects low contrast
+- **GIVEN** preprocessing mode is "auto"
+- **WHEN** image contrast (standard deviation) is below 40
+- **THEN** the system SHALL automatically enable CLAHE contrast enhancement
+
+#### Scenario: Auto mode detects faint edges
+- **GIVEN** preprocessing mode is "auto"
+- **WHEN** image edge strength (Sobel gradient mean) is below 15
+- **THEN** the system SHALL automatically enable sharpening
+
+#### Scenario: Manual mode uses user-specified settings
+- **GIVEN** preprocessing mode is set to "manual"
+- **WHEN** processing begins
+- **THEN** the system SHALL use the user-provided preprocessing configuration
+- **AND** ignore automatic quality analysis
+
+#### Scenario: Disabled mode skips preprocessing
+- **GIVEN** preprocessing mode is set to "disabled"
+- **WHEN** processing begins
+- **THEN** the system SHALL skip all preprocessing
+- **AND** PP-Structure SHALL receive the original image directly
+
+### Requirement: Preprocessing Preview API
+
+The system SHALL provide a preview endpoint that allows users to compare original and preprocessed images before processing.
+
+#### Scenario: Preview returns comparison images
+- **GIVEN** a task with uploaded document
+- **WHEN** user requests preprocessing preview for a specific page
+- **THEN** the system SHALL return URLs or data for both original and preprocessed images
+- **AND** user can visually compare the difference
+
+#### Scenario: Preview shows auto-detected settings
+- **GIVEN** preview is requested with mode "auto"
+- **WHEN** the system analyzes the page
+- **THEN** the response SHALL include the auto-detected preprocessing configuration
+- **AND** include quality metrics (contrast, edge_strength)
+
+#### Scenario: Preview accepts manual configuration
+- **GIVEN** preview is requested with mode "manual"
+- **WHEN** user provides specific preprocessing settings
+- **THEN** the system SHALL apply those settings to generate preview
+- **AND** return the preprocessed result for user verification
+
 ### Requirement: Preprocessing Track Isolation
 
 The layout preprocessing feature SHALL only affect layout detection input without impacting other processing components.
@@ -53,3 +102,27 @@ The layout preprocessing feature SHALL only affect layout detection input withou
 - **WHEN** layout detection completes
 - **THEN** the preprocessed image SHALL NOT be persisted to storage
 - **AND** only the original image and element crops SHALL be saved
+
+### Requirement: Preprocessing Frontend UI
+
+The frontend SHALL provide a user interface for configuring and previewing preprocessing settings.
+
+#### Scenario: Mode selection is available
+- **GIVEN** the user is configuring OCR track processing
+- **WHEN** the preprocessing settings panel is displayed
+- **THEN** the user SHALL be able to select mode: Auto (default), Manual, or Disabled
+- **AND** Auto mode SHALL be pre-selected
+
+#### Scenario: Manual mode shows configuration options
+- **GIVEN** the user selects Manual mode
+- **WHEN** the settings panel updates
+- **THEN** the user SHALL see options for:
+  - Contrast enhancement (None / Histogram / CLAHE)
+  - Sharpen toggle
+  - Binarize toggle
+
+#### Scenario: Preview button triggers comparison view
+- **GIVEN** preprocessing settings are configured
+- **WHEN** the user clicks Preview button
+- **THEN** the system SHALL display side-by-side comparison of original and preprocessed images
+- **AND** show detected quality metrics