feat: Add native Django ALTCHA challenge generation and server-side HMAC validation endpoint

backend/apps/events/urls.py

@@ -2,7 +2,7 @@ from django.urls import path, include
 from rest_framework.routers import DefaultRouter
 from .views import (
     SkillViewSet, EventViewSet, TaskAreaViewSet,
-    ShiftViewSet, ShiftSignupView, EventTemplateViewSet
+    ShiftViewSet, ShiftSignupView, EventTemplateViewSet, AltchaChallengeView
 )
 
 router = DefaultRouter()
@@ -13,6 +13,7 @@ router.register('shifts', ShiftViewSet, basename='shift')
 router.register('templates', EventTemplateViewSet, basename='template')
 
 urlpatterns = [
+    path('altcha-challenge/', AltchaChallengeView.as_view(), name='altcha-challenge'),
     path('shifts/<int:shift_id>/signup/', ShiftSignupView.as_view(), name='shift-signup'),
     path('shifts/<int:shift_id>/signup/<int:signup_id>/', ShiftSignupView.as_view(), name='shift-signup-detail'),
     path('', include(router.urls)),