feat: add dual-track API endpoints for document processing

- Add ProcessingTrackEnum, ProcessingOptions, ProcessingMetadata schemas
- Add DocumentAnalysisResponse for document type detection
- Update /start endpoint with dual-track query parameters
- Add /analyze endpoint for document type detection with confidence scores
- Add /metadata endpoint for processing track information
- Add /download/unified endpoint for UnifiedDocument format export
- Update tasks.md to mark Section 6 API updates as completed

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

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

openspec/changes/dual-track-document-processing/tasks.md

@@ -84,18 +84,18 @@
   - [ ] 5.3.3 Handle font substitution for target languages
 
 ## 6. API Updates
-- [ ] 6.1 Update OCR endpoints
-  - [ ] 6.1.1 Add processing_track parameter
-  - [ ] 6.1.2 Support track auto-detection
-  - [ ] 6.1.3 Return processing metadata
-- [ ] 6.2 Add document type detection endpoint
-  - [ ] 6.2.1 Create /analyze endpoint
-  - [ ] 6.2.2 Return recommended processing track
-  - [ ] 6.2.3 Provide confidence scores
-- [ ] 6.3 Update result export endpoints
-  - [ ] 6.3.1 Support UnifiedDocument format
-  - [ ] 6.3.2 Add format conversion options
-  - [ ] 6.3.3 Include processing track information
+- [x] 6.1 Update OCR endpoints
+  - [x] 6.1.1 Add processing_track parameter
+  - [x] 6.1.2 Support track auto-detection
+  - [x] 6.1.3 Return processing metadata
+- [x] 6.2 Add document type detection endpoint
+  - [x] 6.2.1 Create /analyze endpoint
+  - [x] 6.2.2 Return recommended processing track
+  - [x] 6.2.3 Provide confidence scores
+- [x] 6.3 Update result export endpoints
+  - [x] 6.3.1 Support UnifiedDocument format
+  - [x] 6.3.2 Add format conversion options
+  - [x] 6.3.3 Include processing track information
 
 ## 7. Frontend Updates
 - [ ] 7.1 Update task detail view