Bump django from 5.1.1 to 5.1.5 in /backend

Bumps [django](https://github.com/django/django) from 5.1.1 to 5.1.5.
- [Commits](django/django@5.1.1...5.1.5)

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

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