fix: sanitize filenames in upload paths to prevent path traversal (#8879)

* fix: sanitize filenames in upload paths to prevent path traversal (GHSA-v57h-5999-w7xp)

Add server-side filename sanitization across all file upload endpoints
to prevent path traversal sequences (../) in user-supplied filenames
from being incorporated into S3 object keys. While S3 keys are flat
strings and not vulnerable to filesystem traversal, this adds
defense-in-depth and prevents S3 key pollution.

Changes:
- Add sanitize_filename() utility in path_validator.py
- Sanitize filenames in get_upload_path() for FileAsset and IssueAttachment models
- Sanitize name parameter in all upload view endpoints

* fix: address PR review feedback on filename sanitization

- Remove unused `import re`
- Normalize backslashes to forward slashes before os.path.basename()
  so Windows-style paths (e.g. ..\..\..\evil.txt) are handled on POSIX
- Strip whitespace before removing leading dots so " .env" is caught
- Return None instead of "unnamed" for empty input so existing
  `if not name` validation guards remain effective
- Add `or "unnamed"` fallback at call sites that lack a name guard

* fix: use random hex name as fallback in get_upload_path instead of "unnamed"

* fix: resolve ruff E501 line too long in DuplicateAssetEndpoint