Bump django from 5.2.14 to 5.2.15 in /backend

Bumps [django](https://github.com/django/django) from 5.2.14 to 5.2.15.
- [Commits](django/django@5.2.14...5.2.15)

---
updated-dependencies:
- dependency-name: django
  dependency-version: 5.2.15
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/pyproject.toml

@@ -7,7 +7,7 @@ requires-python = ">=3.13"
 dependencies = [
   "blessed~=1.39.0",
   "cryptography~=48.0.0",
-  "django~=5.2.0",
+  "django~=5.2.15",
   "django-allauth~=65.16.0",
   "django-auditlog~=3.4.0",
   "django-environ~=0.13.0",