feat(notifications): add ntfy as a first-class notification channel

Adds ntfy.sh (and self-hosted instances) as a new push notification
channel with full parity to the existing webhook channel.

- Backend: NtfyConfig type, getUserNtfyConfig, getAdminNtfyConfig,
  resolveNtfyUrl, sendNtfy (header-based API with Title/Priority/Tags/
  Click headers), testNtfy, NTFY_EVENT_META (priority + emoji tags per
  event), SSRF guard via existing checkSsrf + createPinnedDispatcher
- notificationPreferencesService: ntfy added to NotifChannel union,
  IMPLEMENTED_COMBOS, getActiveChannels parser, getAvailableChannels,
  ADMIN_GLOBAL_CHANNELS, and AvailableChannels interface
- notificationService: per-user ntfy dispatch after webhook block;
  admin-scoped ntfy via getAdminGlobalPref for version_available events
- Routes: POST /api/notifications/test-ntfy with saved-token fallback
- authService: admin_ntfy_server/topic/token in ADMIN_SETTINGS_KEYS,
  masked + encrypted on read/write
- settingsService: ntfy_token added to ENCRYPTED_SETTING_KEYS
- Frontend: ntfy topic/server/token inputs + Save/Test/Clear buttons in
  NotificationsTab; admin Ntfy panel in AdminPage; testNtfy API method
- i18n: full English strings; English placeholders in 14 other locales
- Tests: resolveNtfyUrl, sendNtfy, dispatch integration, UI tests,
  MSW handler for test-ntfy endpoint

client/tests/helpers/msw/handlers/notifications.ts

@@ -61,6 +61,10 @@ export const notificationHandlers = [
     return HttpResponse.json({ success: true });
   }),
 
+  http.post('/api/notifications/test-ntfy', async () => {
+    return HttpResponse.json({ success: true });
+  }),
+
   http.post('/api/notifications/in-app/:id/respond', async ({ request, params }) => {
     const body = await request.json() as { response: string };
     return HttpResponse.json({