build(deps): bump zstandard from 0.18.0 to 0.19.0 in /backend

Bumps [zstandard](https://github.com/indygreg/python-zstandard) from 0.18.0 to 0.19.0.
- [Release notes](https://github.com/indygreg/python-zstandard/releases)
- [Changelog](https://github.com/indygreg/python-zstandard/blob/main/docs/news.rst)
- [Commits](indygreg/python-zstandard@0.18.0...0.19.0)

---
updated-dependencies:
- dependency-name: zstandard
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/requirements.txt

@@ -11,4 +11,4 @@ python-dateutil==2.8.2
 pytoml==0.1.21
 redis==4.3.4
 swagger-ui-bundle==0.0.9
-zstandard==0.18.0
+zstandard==0.19.0