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/integrations/qbittorrent.service.ts

@@ -729,6 +729,26 @@ export class QBittorrentService implements IDownloadClient {
     }
   }
 
+  /**
+   * Get all configured categories from qBittorrent
+   */
+  async getCategories(): Promise<string[]> {
+    if (!this.cookie) {
+      await this.login();
+    }
+
+    try {
+      const response = await this.client.get('/torrents/categories', {
+        headers: { Cookie: this.cookie },
+      });
+
+      return Object.keys(response.data || {});
+    } catch (error) {
+      logger.error('Failed to get categories', { error: error instanceof Error ? error.message : String(error) });
+      return [];
+    }
+  }
+
   /**
    * Set category for torrent
    */