fix(admin): series Delete button actually deletes the series

The /admin/series Delete handler only PUT-unlinked each event and never
called the DELETE /api/admin/series/[id] endpoint, so the series document
persisted (a no-op for empty series). Replace the redundant per-event loop
with a single DELETE call — the endpoint already unlinks events server-side.
Unskip the e2e delete test.

app/pages/admin/series/index.vue

@@ -604,15 +604,7 @@ const deleteSeries = (series) => {
   confirmAction.execute = async () => {
     confirmAction.running = true
     try {
-      for (const event of series.events) {
-        await $fetch(`/api/admin/events/${event.id}`, {
-          method: 'PUT',
-          body: {
-            ...event,
-            series: { isSeriesEvent: false, id: '', title: '', description: '', type: 'workshop_series', position: 1, totalEvents: null },
-          },
-        })
-      }
+      await $fetch(`/api/admin/series/${series.id}`, { method: 'DELETE' })
       confirmAction.show = false
       await refresh()
     } catch (error) {