feat: allow the exclusion of inactive schedules in the REST API

This filter is currently applied programmatically in the dashboard. It
makes much more sense to do this on a database level.

refs dashboard#246

program/filters.py

@@ -170,6 +170,19 @@ class ScheduleFilterSet(filters.FilterSet):
         field_name="show",
         help_text="Return only schedules that belong to the specified show(s).",
     )
+    exclude_inactive = filters.BooleanFilter(
+        method="filter_exclude_inactive",
+        help_text="Excludes all schedules that don’t have timeslots in the future.",
+    )
+
+    def filter_exclude_inactive(self, queryset: QuerySet, name: str, value: bool):
+        if not value:
+            return queryset
+        return queryset.filter(
+            Exists(
+                models.TimeSlot.objects.filter(schedule=OuterRef("pk"), end__gte=timezone.now())
+            )
+        )
 
 
 class TimeSlotFilterSet(filters.FilterSet):