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.

src/app/setup/steps/DownloadClientStep.tsx

@@ -5,7 +5,7 @@
 
 'use client';
 
-import { useState, useEffect } from 'react';
+import { useState } from 'react';
 import { Button } from '@/components/ui/Button';
 import { DownloadClientManagement } from '@/components/admin/download-clients/DownloadClientManagement';
 import { DownloadClientType } from '@/lib/interfaces/download-client.interface';
@@ -24,6 +24,7 @@ interface DownloadClient {
   localPath?: string;
   category?: string;
   customPath?: string;
+  postImportCategory?: string;
 }
 
 interface DownloadClientStepProps {