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.

Changes:
- Defined `VALID_FILENAME_PATTERN` and `VALID_FILENAME_RE` in `app/utils/filename_utils.py`.
- Updated `app/tasks/extract_metadata_with_gpt.py` to use `VALID_FILENAME_RE`.
- Updated `tests/test_path_traversal_security.py` to use `VALID_FILENAME_PATTERN`.

This refactoring addresses the duplication mentioned in the TODO in `tests/test_path_traversal_security.py`.

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 = [