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/alembic/versions/e51c9a16ee16_increase_file_type_column_length.py

@@ -0,0 +1,40 @@
+"""increase_file_type_column_length
+
+Revision ID: e51c9a16ee16
+Revises: 4d37f412d37a
+Create Date: 2025-11-30 15:03:28.950186
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'e51c9a16ee16'
+down_revision: Union[str, None] = '4d37f412d37a'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    op.alter_column(
+        'tool_ocr_tasks',
+        'file_type',
+        existing_type=sa.String(50),
+        type_=sa.String(100),
+        existing_nullable=True
+    )
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.alter_column(
+        'tool_ocr_tasks',
+        'file_type',
+        existing_type=sa.String(100),
+        type_=sa.String(50),
+        existing_nullable=True
+    )