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/transmission.service.ts

@@ -441,6 +441,29 @@ export class TransmissionService implements IDownloadClient {
     // No-op: torrents are managed by the seeding cleanup scheduler
   }
 
+  /**
+   * Get available categories/labels.
+   * Transmission uses free-form labels — no predefined list to fetch.
+   */
+  async getCategories(): Promise<string[]> {
+    return [];
+  }
+
+  /**
+   * Set the label for a torrent.
+   * Uses the torrent-set RPC method to replace the labels array.
+   */
+  async setCategory(id: string, category: string): Promise<void> {
+    try {
+      const torrent = await this.getTorrentByHash(id);
+      await this.rpc('torrent-set', { ids: [torrent.hashString], labels: [category] });
+      logger.info(`Set label for torrent ${id}: ${category}`);
+    } catch (error) {
+      logger.error('Failed to set label', { error: error instanceof Error ? error.message : String(error) });
+      throw new Error('Failed to set torrent label');
+    }
+  }
+
   // =========================================================================
   // Internal Helpers
   // =========================================================================