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/apprise.provider.ts

@@ -0,0 +1,133 @@
+/**
+ * Component: Apprise Notification Provider
+ * Documentation: documentation/backend/services/notifications.md
+ */
+
+import { INotificationProvider, NotificationPayload, ProviderMetadata } from '../INotificationProvider';
+
+export interface AppriseConfig {
+  serverUrl: string;
+  urls?: string;
+  configKey?: string;
+  tag?: string;
+  authToken?: string;
+}
+
+// Apprise notification types by event
+const APPRISE_TYPES: Record<string, string> = {
+  request_pending_approval: 'info',
+  request_approved: 'success',
+  request_available: 'success',
+  request_error: 'failure',
+};
+
+export class AppriseProvider implements INotificationProvider {
+  type = 'apprise' as const;
+  sensitiveFields = ['urls', 'authToken'];
+  metadata: ProviderMetadata = {
+    type: 'apprise',
+    displayName: 'Apprise',
+    description: 'Send notifications via Apprise API to 100+ services',
+    iconLabel: 'A',
+    iconColor: 'bg-purple-500',
+    configFields: [
+      { name: 'serverUrl', label: 'Server URL', type: 'text', required: true, placeholder: 'http://apprise:8000' },
+      { name: 'urls', label: 'Notification URLs', type: 'password', required: false, placeholder: 'slack://token, discord://webhook_id/token, ...' },
+      { name: 'configKey', label: 'Config Key', type: 'text', required: false, placeholder: 'Persistent configuration key' },
+      { name: 'tag', label: 'Tag', type: 'text', required: false, placeholder: 'Filter tag for stateful config' },
+      { name: 'authToken', label: 'Auth Token', type: 'password', required: false, placeholder: 'Optional API auth token' },
+    ],
+  };
+
+  async send(config: Record<string, any>, payload: NotificationPayload): Promise<void> {
+    const appriseConfig = config as unknown as AppriseConfig;
+    const { title, body } = this.formatMessage(payload);
+
+    const serverUrl = appriseConfig.serverUrl.replace(/\/+$/, '');
+    const notificationType = APPRISE_TYPES[payload.event] || 'info';
+
+    const headers: Record<string, string> = {
+      'Content-Type': 'application/json',
+    };
+
+    if (appriseConfig.authToken) {
+      headers['Authorization'] = `Bearer ${appriseConfig.authToken}`;
+    }
+
+    // Stateful mode: use configKey endpoint
+    if (appriseConfig.configKey) {
+      const url = `${serverUrl}/notify/${appriseConfig.configKey}`;
+      const requestBody: Record<string, string> = {
+        title,
+        body,
+        type: notificationType,
+      };
+
+      if (appriseConfig.tag) {
+        requestBody.tag = appriseConfig.tag;
+      }
+
+      const response = await fetch(url, {
+        method: 'POST',
+        headers,
+        body: JSON.stringify(requestBody),
+      });
+
+      if (!response.ok) {
+        const errorText = await response.text().catch(() => 'Unknown error');
+        throw new Error(`Apprise API failed: ${response.status} ${errorText}`);
+      }
+      return;
+    }
+
+    // Stateless mode: send URLs directly
+    if (!appriseConfig.urls) {
+      throw new Error('Apprise requires either notification URLs or a config key');
+    }
+
+    const url = `${serverUrl}/notify/`;
+    const requestBody = {
+      urls: appriseConfig.urls,
+      title,
+      body,
+      type: notificationType,
+    };
+
+    const response = await fetch(url, {
+      method: 'POST',
+      headers,
+      body: JSON.stringify(requestBody),
+    });
+
+    if (!response.ok) {
+      const errorText = await response.text().catch(() => 'Unknown error');
+      throw new Error(`Apprise API failed: ${response.status} ${errorText}`);
+    }
+  }
+
+  private formatMessage(payload: NotificationPayload): { title: string; body: string } {
+    const { event, title, author, userName, message } = payload;
+
+    const eventTitles: Record<string, string> = {
+      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],
+      body: messageLines.join('\n'),
+    };
+  }
+}