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/src/api/client.ts

@@ -486,6 +486,7 @@ export const notificationsApi = {
   updatePreferences: (prefs: Record<string, Record<string, boolean>>) => apiClient.put('/notifications/preferences', prefs).then(r => r.data),
   testSmtp: (email?: string) => apiClient.post('/notifications/test-smtp', { email }).then(r => r.data),
   testWebhook: (url?: string) => apiClient.post('/notifications/test-webhook', { url }).then(r => r.data),
+  testNtfy: (payload: { topic?: string; server?: string | null; token?: string | null }) => apiClient.post('/notifications/test-ntfy', payload).then(r => r.data),
 }
 
 export const inAppNotificationsApi = {