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>

app/utils/filename_utils.py

@@ -8,6 +8,11 @@ from pathlib import Path
 
 logger = logging.getLogger(__name__)
 
+# Pattern for valid filenames (alphanumeric, dash, underscore, period, and space)
+# Used for validating GPT-provided filenames and other inputs
+VALID_FILENAME_PATTERN = r"^[\w\-\. ]+$"
+VALID_FILENAME_RE = re.compile(VALID_FILENAME_PATTERN)
+
 
 def get_unique_filename(original_path: str, check_exists_func: Callable[[str], bool] | None = None) -> str:
     """