Add cancelled field to event slots

Make it possible to represent one slot out of a multi-slot event being
cancelled by adding a field for it in the slot, in addition to the
existing field on the event itself.

shared/types/api.ts

@@ -81,6 +81,7 @@ export const apiScheduleEventSlotSchema = z.object({
 	start: z.string(),
 	end: z.string(),
 	locationIds: z.array(idSchema),
+	cancelled: z.optional(z.boolean()),
 	assigned: z.optional(z.array(z.number())),
 	interested: z.optional(z.number()),
 });