chore(deps): bump django from 4.2.14 to 4.2.15 in /backend

Bumps [django](https://github.com/django/django) from 4.2.14 to 4.2.15.
- [Commits](django/django@4.2.14...4.2.15)

---
updated-dependencies:
- dependency-name: django
  dependency-type: direct:production
...

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

backend/requirements.txt

@@ -1,4 +1,4 @@
-Django==4.2.14
+Django==4.2.15
 structlog==24.1.0
 django-structlog==8.1.0
 celery==5.3.6