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/models/task.py

@@ -36,7 +36,7 @@ class Task(Base):
     task_id = Column(String(255), unique=True, nullable=False, index=True,
                     comment="Unique task identifier (UUID)")
     filename = Column(String(255), nullable=True, index=True)
-    file_type = Column(String(50), nullable=True)
+    file_type = Column(String(100), nullable=True)
     status = Column(SQLEnum(TaskStatus), default=TaskStatus.PENDING, nullable=False,
                    index=True)
     result_json_path = Column(String(500), nullable=True,