feat: add unified JSON export with standardized schema

- Create JSON Schema definition for UnifiedDocument format
- Implement UnifiedDocumentExporter service with multiple export formats
- Include comprehensive processing metadata and statistics
- Update OCR service to use new exporter for dual-track outputs
- Support JSON, Markdown, Text, and legacy format exports

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

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

backend/app/services/__init__.py

@@ -1,3 +1,17 @@
 """
 Tool_OCR - Services Package
 """
+
+from .unified_document_exporter import (
+    UnifiedDocumentExporter,
+    ExportFormat,
+    JSONSchemaValidator,
+    save_unified_document
+)
+
+__all__ = [
+    'UnifiedDocumentExporter',
+    'ExportFormat',
+    'JSONSchemaValidator',
+    'save_unified_document'
+]