fix: improve OCR track multi-line text rendering and HTML table detection

Multi-line text rendering (pdf_generator_service.py):
- Calculate font size by dividing bbox height by number of lines
- Start Y coordinate from bbox TOP instead of bottom
- Use non_empty_lines for proper line positioning

HTML table detection:
- pp_structure_enhanced.py: Detect HTML tables in 'text' type content
  and reclassify to TABLE when <table tag found
- pdf_generator_service.py: Content-based reclassification from TEXT
  to TABLE during UnifiedDocument parsing
- ocr_to_unified_converter.py: Fallback to check 'content' field for
  HTML tables when 'html' field is empty

Known issue: OCR processing still has quality issues that need further
investigation and fixes.

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

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

backend/app/services/pp_structure_enhanced.py

@@ -286,6 +286,15 @@ class PPStructureEnhanced:
                 elif isinstance(res, str):
                     content = res
 
+            # Content-based HTML table detection: PP-StructureV3 sometimes
+            # classifies tables as 'text' but returns HTML table content
+            html_table_content = None
+            if content and '<table' in content.lower():
+                if mapped_type == ElementType.TEXT or element_type == 'text':
+                    logger.info(f"Element {idx}: Detected HTML table content in 'text' type, reclassifying to TABLE")
+                    mapped_type = ElementType.TABLE
+                    html_table_content = content  # Store for later use
+
             # Create element
             element = {
                 'element_id': f"pp3_{current_page}_{idx}",
@@ -300,12 +309,13 @@ class PPStructureEnhanced:
 
             # Special handling for tables
             if mapped_type == ElementType.TABLE:
-                # Extract table structure if available
-                if 'res' in item and isinstance(item['res'], dict):
+                # Use HTML content from content-based detection or extract from 'res'
+                html_content = html_table_content  # From content-based detection
+                if not html_content and 'res' in item and isinstance(item['res'], dict):
                     html_content = item['res'].get('html', '')
-                    if html_content:
-                        element['html'] = html_content
-                        element['extracted_text'] = self._extract_text_from_html(html_content)
+                if html_content:
+                    element['html'] = html_content
+                    element['extracted_text'] = self._extract_text_from_html(html_content)
 
             # Special handling for images/figures
             elif mapped_type in [ElementType.IMAGE, ElementType.FIGURE]: