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/lib/services/notification/providers/ntfy.provider.ts

@@ -0,0 +1,109 @@
+/**
+ * Component: ntfy Notification Provider
+ * Documentation: documentation/backend/services/notifications.md
+ */
+
+import { INotificationProvider, NotificationPayload, ProviderMetadata } from '../INotificationProvider';
+
+export interface NtfyConfig {
+  serverUrl?: string;
+  topic: string;
+  accessToken?: string;
+  priority?: number;
+}
+
+const DEFAULT_SERVER_URL = 'https://ntfy.sh';
+
+// ntfy priorities by event type (1=min, 2=low, 3=default, 4=high, 5=urgent)
+const NTFY_PRIORITIES = {
+  request_pending_approval: 3, // Default
+  request_approved: 3, // Default
+  request_available: 4, // High
+  request_error: 4, // High
+};
+
+// ntfy tags (emojis) by event type
+const NTFY_TAGS = {
+  request_pending_approval: ['mailbox_with_mail'],
+  request_approved: ['white_check_mark'],
+  request_available: ['tada'],
+  request_error: ['x'],
+};
+
+export class NtfyProvider implements INotificationProvider {
+  type = 'ntfy' as const;
+  sensitiveFields = ['accessToken'];
+  metadata: ProviderMetadata = {
+    type: 'ntfy',
+    displayName: 'ntfy',
+    description: 'Send notifications via ntfy pub/sub',
+    iconLabel: 'N',
+    iconColor: 'bg-teal-500',
+    configFields: [
+      { name: 'serverUrl', label: 'Server URL', type: 'text', required: false, placeholder: 'https://ntfy.sh', defaultValue: 'https://ntfy.sh' },
+      { name: 'topic', label: 'Topic', type: 'text', required: true, placeholder: 'readmeabook' },
+      { name: 'accessToken', label: 'Access Token', type: 'password', required: false, placeholder: 'tk_...' },
+    ],
+  };
+
+  async send(config: Record<string, any>, payload: NotificationPayload): Promise<void> {
+    const ntfyConfig = config as unknown as NtfyConfig;
+    const { title, message } = this.formatMessage(payload);
+
+    const serverUrl = (ntfyConfig.serverUrl || DEFAULT_SERVER_URL).replace(/\/+$/, '');
+    const url = `${serverUrl}/${ntfyConfig.topic}`;
+
+    const headers: Record<string, string> = {
+      'Content-Type': 'application/json',
+    };
+
+    if (ntfyConfig.accessToken) {
+      headers['Authorization'] = `Bearer ${ntfyConfig.accessToken}`;
+    }
+
+    const body = {
+      topic: ntfyConfig.topic,
+      title,
+      message,
+      priority: ntfyConfig.priority ?? NTFY_PRIORITIES[payload.event],
+      tags: NTFY_TAGS[payload.event],
+    };
+
+    const response = await fetch(url, {
+      method: 'POST',
+      headers,
+      body: JSON.stringify(body),
+    });
+
+    if (!response.ok) {
+      const errorText = await response.text().catch(() => 'Unknown error');
+      throw new Error(`ntfy API failed: ${response.status} ${errorText}`);
+    }
+  }
+
+  private formatMessage(payload: NotificationPayload): { title: string; message: string } {
+    const { event, title, author, userName, message } = payload;
+
+    const eventTitles = {
+      request_pending_approval: 'New Request Pending Approval',
+      request_approved: 'Request Approved',
+      request_available: 'Audiobook Available',
+      request_error: 'Request Error',
+    };
+
+    const messageLines = [
+      `📚 ${title}`,
+      `✍️ ${author}`,
+      `👤 Requested by: ${userName}`,
+    ];
+
+    if (message) {
+      messageLines.push(`⚠️ Error: ${message}`);
+    }
+
+    return {
+      title: eventTitles[event],
+      message: messageLines.join('\n'),
+    };
+  }
+}