fix: resolve E2E test failures and add Office direct extraction design

- Fix MySQL connection timeout by creating fresh DB session after OCR
- Fix /analyze endpoint attribute errors (detect vs analyze, metadata)
- Add processing_track field extraction to TaskDetailResponse
- Update E2E tests to use POST for /analyze endpoint
- Increase Office document timeout to 300s
- Add Section 2.4 tasks for Office document direct extraction
- Document Office → PDF → Direct track strategy in design.md

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

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

backend/app/services/ocr_service.py

@@ -27,7 +27,7 @@ try:
     from app.models.unified_document import (
         UnifiedDocument, DocumentMetadata,
         ProcessingTrack, ElementType, DocumentElement, Page, Dimensions,
-        BoundingBox, ProcessingInfo
+        BoundingBox
     )
     DUAL_TRACK_AVAILABLE = True
 except ImportError as e: