🚀 feat: upgrade to Django 6.0

Upgrade Django from 5.2.9 to 6.0 by updating requirements files.

- Update requirements/main.txt: Django 5.2.9 → 6.0
- Update requirements/dev.txt: Django 5.2.9 → 6.0
- All 33 tests pass successfully
- Wagtail 7.2.1 supports Django 6.0

Django 6.0 was released on December 3, 2025:
https://docs.djangoproject.com/en/6.0/releases/6.0/

Wagtail 7.2.x supports Django 6.0:
wagtail/wagtail#13622

Closes #173

Author: matrixise

requirements/dev.txt

@@ -24,7 +24,7 @@ defusedxml==0.7.1
     # via
     #   -c requirements/main.txt
     #   py-serializable
-django==5.2.9
+django==6.0
     # via
     #   -c requirements/main.txt
     #   django-debug-toolbar

requirements/main.txt

@@ -32,7 +32,7 @@ dj-database-url==3.0.1
     # via -r requirements/main.in
 dj-static==0.0.6
     # via -r requirements/main.in
-django==5.2.9
+django==6.0
     # via
     #   -r requirements/main.in
     #   dj-database-url