fix: improve Office document processing with Direct track

- Force Office documents (PPTX, DOCX, XLSX) to use Direct track after
  LibreOffice conversion, since converted PDFs always have extractable text
- Fix PDF generator to not exclude text in image regions for Direct track,
  allowing text to render on top of background images (critical for PPT)
- Increase file_type column from VARCHAR(50) to VARCHAR(100) to support
  long MIME types like PPTX
- Remove reference to non-existent total_images metadata attribute

This significantly improves processing time for Office documents
(from ~170s OCR to ~10s Direct) while preserving text quality.

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

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

backend/app/services/direct_extraction_engine.py

@@ -1317,8 +1317,6 @@ class DirectExtractionEngine:
             doc.close()
 
             if images_added > 0:
-                current_images = unified_doc.metadata.total_images or 0
-                unified_doc.metadata.total_images = current_images + images_added
                 logger.info(f"Added {images_added} inline image regions to document")
 
         except Exception as e: