chore: bump ruff

Bumps the minor-and-patch group with 1 update in the /backend directory: [ruff](https://github.com/astral-sh/ruff).


Updates `ruff` from 0.15.20 to 0.16.3
- [Release notes](https://github.com/astral-sh/ruff/releases)
- [Changelog](https://github.com/astral-sh/ruff/blob/main/CHANGELOG.md)
- [Commits](astral-sh/ruff@0.15.20...0.16.3)

---
updated-dependencies:
- dependency-name: ruff
  dependency-version: 0.15.21
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: minor-and-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/pyproject.toml

@@ -271,7 +271,7 @@ dev = [
     # formatter's output, which can shift between ruff releases. A floating
     # spec let CI pick a newer ruff than contributors ran locally, so formatted
     # code still failed the check. Bump this deliberately when adopting a new ruff.
-    "ruff==0.15.20",
+    "ruff==0.16.3",
     "mypy>=1.13.0",
     "pre-commit>=4.0.0",
     "pyinstaller>=6.0.0",