Bump django from 5.2.7 to 5.2.15 in /backend

Bumps [django](https://github.com/django/django) from 5.2.7 to 5.2.15.
- [Commits](django/django@5.2.7...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/requirements.in

@@ -1,4 +1,4 @@
-Django>=5.2
+Django>=5.2.15
 djangorestframework
 dj-rest-auth[with_social]
 django-livereload-server

backend/requirements.txt

@@ -55,10 +55,17 @@ defusedxml==0.7.1
     # via
     #   djangosaml2
     #   pysaml2
-    #   python3-openid
-django==5.2.7
 dj-rest-auth[with-social]==7.2.0
     # via -r requirements.in
+django==5.2.15
+    # via
+    #   -r requirements.in
+    #   dj-rest-auth
+    #   django-allauth
+    #   django-livereload-server
+    #   django-revproxy
+    #   djangorestframework
+    #   djangosaml2
 django-allauth[socialaccount]==65.15.1
     # via dj-rest-auth
 django-livereload-server==0.5.1