Add extensible notification providers + UI/API

Introduce a provider-based notification system and wire it through the API and admin UI. Added INotificationProvider + notification service implementation and providers (apprise, discord, ntfy, pushover), plus a GET /api/admin/notifications/providers endpoint to expose provider metadata. Refactored code to use provider type strings (removed enum coupling), updated masking/encryption calls, and simplified the test notification endpoint to accept backendId or type+config and call sendToBackend directly.

UI: NotificationsTab now fetches provider metadata and renders provider cards and dynamic config forms (fields driven by provider metadata). Added config field rendering, improved backend cards, and edit/delete actions.

APIs: New providers route, updated admin notification CRUD routes to validate provider types dynamically, updated test route schema. Added download-client categories POST API to fetch categories from clients and wired postImportCategory handling in download-client routes.

Other notable changes: BookDate now fetches Claude models dynamically from Anthropic's Models API; added paginated model fetch helper. Added ALLOW_WEAK_PASSWORD flag exposure to auth providers and password change logic. Doc updates and various tests added/updated. File-organization doc clarifies EPERM fix using stream-based copy.

tests/api/admin-notifications.routes.test.ts

@@ -35,8 +35,9 @@ vi.mock('@/lib/middleware/auth', () => ({
   requireAdmin: requireAdminMock,
 }));
 
-vi.mock('@/lib/services/notification.service', () => ({
+vi.mock('@/lib/services/notification', () => ({
   getNotificationService: () => notificationServiceMock,
+  getRegisteredProviderTypes: () => ['discord', 'ntfy', 'pushover'],
 }));
 
 describe('Admin notifications routes', () => {