Fix admin login 500 when clients omit the trailing slash.

Co-authored-by: Cursor <cursoragent@cursor.com>

api/urls.py

@@ -1,5 +1,6 @@
-from django.urls import path, include
+from django.urls import include, re_path
 from rest_framework.routers import DefaultRouter
+
 from .views import (
     ContactUsViewSet,
     CompositionViewSet,
@@ -9,14 +10,20 @@ from .views import (
     admin_me,
 )
 
-router = DefaultRouter()
+
+class OptionalSlashRouter(DefaultRouter):
+    """Match /api/resource and /api/resource/ so POST clients do not hit APPEND_SLASH."""
+    trailing_slash = '/?'
+
+
+router = OptionalSlashRouter()
 router.register(r'contact-us', ContactUsViewSet, basename='contact-us')
 router.register(r'compositions', CompositionViewSet, basename='composition')
 router.register(r'campaigns', CampaignViewSet, basename='campaign')
 
 urlpatterns = [
-    path('admin/login/', admin_login, name='admin-login'),
-    path('admin/logout/', admin_logout, name='admin-logout'),
-    path('admin/me/', admin_me, name='admin-me'),
-    path('', include(router.urls)),
+    re_path(r'^admin/login/?$', admin_login, name='admin-login'),
+    re_path(r'^admin/logout/?$', admin_logout, name='admin-logout'),
+    re_path(r'^admin/me/?$', admin_me, name='admin-me'),
+    re_path(r'^', include(router.urls)),
 ]