refactor(tasks): extract filename regex to shared constant

Move the valid filename regex pattern to a shared constant in `app/utils/filename_utils.py` and update both the task logic and security tests to use it. This eliminates duplication and ensures consistency across the codebase.

Also normalized line endings to LF in affected files to ensure CI compatibility.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/test_path_traversal_security.py

@@ -212,9 +212,10 @@ class TestExtractMetadataFilenameValidation:
         """Test that invalid filename formats are rejected."""
         import re
 
-        # Valid pattern from extract_metadata_with_gpt.py
-        # TODO: Consider extracting this to a shared constant to avoid duplication
-        valid_pattern = r"^[\w\-\. ]+$"
+        from app.utils.filename_utils import VALID_FILENAME_PATTERN
+
+        # Valid pattern from app.utils.filename_utils
+        valid_pattern = VALID_FILENAME_PATTERN
 
         # Test valid filenames
         valid_filenames = [