test: add unit tests for DocumentTypeDetector

- Create test directory structure for backend
- Add pytest fixtures for test files (PDF, images, Office docs)
- Add 20 unit tests covering:
  - PDF type detection (editable, scanned, mixed)
  - Image file detection (PNG, JPG)
  - Office document detection (DOCX)
  - Text file detection
  - Edge cases (file not found, unknown types)
  - Batch processing and statistics
- Mark tasks 1.1.4 and 1.3.5 as completed in tasks.md

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

.gitignore

@@ -97,4 +97,6 @@ storage/results/*
 *.log
 __pycache__/
 *.bak
+# Ignore temporary test files in root, but allow backend/tests/
 test_*.py
+!backend/tests/**/test_*.py