feat(notify): own webhook as a full notification channel

The custom webhook could be entered but never switched on since the
notification matrix replaced the old alarm channels. It is now a third
column next to e-mail and Discord for every event and the daily summary;
bundles go out as JSON (event "digest").

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/settings/__tests__/UserSettingsView.alarms.test.tsx

@@ -88,19 +88,53 @@ describe('UserSettingsView notification matrix', () => {
           paused: true,
           includeOwnWatched: true,
           events: {
-            due: { email: true, discord: true },
-            shared: { email: true, discord: false },
-            taskDone: { email: true, discord: false },
-            watched: { email: true, discord: false },
+            due: { email: true, discord: true, webhook: false },
+            shared: { email: true, discord: false, webhook: false },
+            taskDone: { email: true, discord: false, webhook: false },
+            watched: { email: true, discord: false, webhook: false },
           },
           dailySummary: {
             enabled: true,
             time: '08:30',
             email: true,
             discord: true,
+            webhook: false,
           },
         },
       })
     );
   });
+
+  it('enables the webhook column once an own webhook URL is entered and saves it as due channel', async () => {
+    render(
+      <UserSettingsView
+        currentUser={baseUser}
+        activeTheme="dark"
+        themes={[]}
+        customThemes={[]}
+        onThemeChange={() => {}}
+        onCustomThemesChange={() => {}}
+        onUserUpdated={() => {}}
+        onBack={() => {}}
+      />
+    );
+    fireEvent.click(screen.getByText('Alarme'));
+    const off = screen.getAllByTitle('Bitte zuerst unten eine eigene Webhook URL eintragen');
+    expect(off.length).toBe(5);
+    expect((off[0] as HTMLButtonElement).disabled).toBe(true);
+
+    fireEvent.change(screen.getByPlaceholderText(/dein-server\.local/), {
+      target: { value: 'https://hooks.example.com/mindsift' },
+    });
+    fireEvent.click(screen.getByTitle('Fällige Aufgaben (sofort) – Webhook'));
+    fireEvent.click(screen.getByTitle('Freigabe erhalten – Webhook'));
+    fireEvent.click(screen.getByRole('button', { name: 'Alarm-Einstellungen speichern' }));
+
+    await waitFor(() => expect(api.updateSettings).toHaveBeenCalledTimes(1));
+    const patch = vi.mocked(api.updateSettings).mock.calls[0][0];
+    expect(patch.customWebhookUrl).toBe('https://hooks.example.com/mindsift');
+    expect(patch.alarmChannels).toEqual(['email', 'webhook']);
+    expect(patch.notificationSettings?.events.due.webhook).toBe(true);
+    expect(patch.notificationSettings?.events.shared.webhook).toBe(true);
+  });
 });