wip: add TableData.from_dict() for OCR track table parsing (incomplete)

Add TableData.from_dict() and TableCell.from_dict() methods to convert
JSON table dicts to proper TableData objects during UnifiedDocument parsing.

Modified _json_to_document_element() to detect TABLE elements with dict
content containing 'cells' key and convert to TableData.

Note: This fix ensures table elements have proper to_html() method available
but the rendered output still needs investigation - tables may still render
incorrectly in OCR track PDFs.

Files changed:
- unified_document.py: Add from_dict() class methods
- pdf_generator_service.py: Convert table dicts during JSON parsing
- Add fix-ocr-track-table-rendering proposal

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

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

backend/app/services/pdf_generator_service.py

@@ -1945,11 +1945,23 @@ class PDFGeneratorService:
                 if child:
                     children.append(child)
 
+            # Process content based on element type
+            content = elem_dict.get('content', '')
+
+            # For TABLE elements, convert dict content to TableData object
+            if elem_type == ElementType.TABLE and isinstance(content, dict) and 'cells' in content:
+                try:
+                    content = TableData.from_dict(content)
+                    logger.debug(f"Converted table dict to TableData: {content.rows}x{content.cols}, {len(content.cells)} cells")
+                except Exception as e:
+                    logger.warning(f"Failed to convert table dict to TableData: {e}")
+                    # Keep original dict as fallback
+
             # Create element
             element = DocumentElement(
                 element_id=elem_dict.get('element_id', ''),
                 type=elem_type,
-                content=elem_dict.get('content', ''),
+                content=content,
                 bbox=bbox,
                 confidence=elem_dict.get('confidence'),
                 style=style,