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.

e2e/admin-series.spec.js

@@ -57,9 +57,34 @@ test.describe('Admin series CRUD', () => {
     await expect(editedCard).toContainText(editedDescription, { timeout: 10000 })
   })
 
-  // Delete is skipped: the series-management page's "Delete" button only
-  // unlinks events from the series via PUT /api/admin/events/:id; it does
-  // not call DELETE /api/admin/series/:id, so the series record remains.
-  // No UI affordance currently exists to remove an empty series.
-  test.skip('delete a series', async () => {})
+  test('delete a series', async ({ adminPage }) => {
+    const suffix = Date.now().toString().slice(-6)
+    const title = `e2e-series-del-${suffix}`
+
+    // --- Create the series to delete ---
+    await adminPage.goto('/admin/series/create')
+    await expect(adminPage.locator('h1')).toContainText('Create New Series')
+    await adminPage
+      .getByPlaceholder('e.g., Cooperative Game Development Fundamentals')
+      .fill(title)
+    await adminPage
+      .getByPlaceholder('Describe what the series covers and its goals')
+      .fill('e2e delete-me series')
+    await adminPage.getByRole('button', { name: 'Create Series' }).click()
+    await adminPage.waitForURL('**/admin/series', { timeout: 15000 })
+
+    const card = adminPage.locator('.series-card', { hasText: title })
+    await expect(card).toBeVisible({ timeout: 10000 })
+
+    // --- Delete (card button → confirm modal) ---
+    await card.getByRole('button', { name: 'Delete' }).click()
+    const confirmModal = adminPage.locator('.modal-overlay', { hasText: 'Delete Series' })
+    await expect(confirmModal).toBeVisible()
+    await confirmModal.getByRole('button', { name: 'Delete', exact: true }).click()
+
+    // --- Series is gone ---
+    await expect(adminPage.locator('.series-card', { hasText: title })).toHaveCount(0, {
+      timeout: 10000,
+    })
+  })
 })