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>

openspec/changes/fix-ocr-track-table-rendering/proposal.md

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+# Fix OCR Track Table Rendering
+
+## Summary
+
+OCR track PDF generation produces tables with incorrect format and layout. Tables appear without proper structure - cell content is misaligned and the visual format differs significantly from the original document. Image placement is correct, but table rendering is broken.
+
+## Problem Statement
+
+When generating PDF from OCR track results (via `scan.pdf` processed by PP-StructureV3), the output tables have:
+1. **Wrong cell alignment** - content not positioned in proper cells
+2. **Missing table structure** - rows/columns don't match original document layout
+3. **Incorrect content distribution** - all content seems to flow linearly instead of maintaining grid structure
+
+Reference: `backend/storage/results/af7c9ee8-60a0-4291-9f22-ef98d27eed52/`
+- Original: `af7c9ee8-60a0-4291-9f22-ef98d27eed52_scan_page_1.png`
+- Generated: `scan_layout.pdf`
+- Result JSON: `scan_result.json` - Tables have correct `{rows, cols, cells}` structure
+
+## Root Cause Analysis
+
+### Issue 1: Table Content Not Converted to TableData Object
+
+In `_json_to_document_element` (pdf_generator_service.py:1952):
+```python
+element = DocumentElement(
+    ...
+    content=elem_dict.get('content', ''),  # Raw dict, not TableData
+    ...
+)
+```
+
+Table elements have `content` as a dict `{rows: 5, cols: 4, cells: [...]}` but it's not converted to a `TableData` object.
+
+### Issue 2: OCR Track HTML Conversion Fails
+
+In `convert_unified_document_to_ocr_data` (pdf_generator_service.py:464-467):
+```python
+elif isinstance(element.content, dict):
+    html_content = element.content.get('html', str(element.content))
+```
+
+Since there's no 'html' key in the cells-based dict, it falls back to `str(element.content)` = `"{'rows': 5, 'cols': 4, ...}"` - invalid HTML.
+
+### Issue 3: Different Table Rendering Paths
+
+- **Direct track** uses `_draw_table_element_direct` which properly handles dict with cells via `_build_rows_from_cells_dict`
+- **OCR track** uses `draw_table_region` which expects HTML strings and fails with dict content
+
+## Proposed Solution
+
+### Option A: Convert dict to TableData during JSON loading (Recommended)
+
+In `_json_to_document_element`, when element type is TABLE and content is a dict with cells, convert it to a `TableData` object:
+
+```python
+# For TABLE elements, convert dict to TableData
+if elem_type == ElementType.TABLE and isinstance(content, dict) and 'cells' in content:
+    content = self._dict_to_table_data(content)
+```
+
+This ensures `element.content.to_html()` works correctly in `convert_unified_document_to_ocr_data`.
+
+### Option B: Fix conversion in convert_unified_document_to_ocr_data
+
+Handle dict with cells properly by converting to HTML:
+
+```python
+elif isinstance(element.content, dict):
+    if 'cells' in element.content:
+        # Convert cells-based dict to HTML
+        html_content = self._cells_dict_to_html(element.content)
+    elif 'html' in element.content:
+        html_content = element.content['html']
+    else:
+        html_content = str(element.content)
+```
+
+## Impact on Hybrid Mode
+
+Hybrid mode uses Direct track rendering (`_generate_direct_track_pdf`) which already handles dict content properly via `_build_rows_from_cells_dict`. The proposed fixes should not affect hybrid mode negatively.
+
+However, testing should verify:
+1. Hybrid mode continues to work with combined Direct + OCR elements
+2. Table rendering quality is consistent across all tracks
+
+## Success Criteria
+
+1. OCR track tables render with correct structure matching original document
+2. Cell content positioned in proper grid locations
+3. Table borders/grid lines visible
+4. No regression in Direct track or Hybrid mode table rendering
+5. All test files (scan.pdf, img1.png, img2.png, img3.png) produce correct output
+
+## Files to Modify
+
+1. `backend/app/services/pdf_generator_service.py`
+   - `_json_to_document_element`: Convert table dict to TableData
+   - `convert_unified_document_to_ocr_data`: Improve dict handling (if Option B)
+
+2. `backend/app/models/unified_document.py` (optional)
+   - Add `TableData.from_dict()` class method for cleaner conversion
+
+## Testing Plan
+
+1. Test scan.pdf with OCR track - verify table structure matches original
+2. Test img1.png, img2.png, img3.png with OCR track
+3. Test PDF files with Direct track - verify no regression
+4. Test Hybrid mode with files that trigger OCR fallback