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/interfaces/download-client.interface.ts

@@ -177,4 +177,22 @@ export interface IDownloadClient {
    * @param id - Download ID
    */
   postProcess(id: string): Promise<void>;
+
+  /**
+   * Get available categories/labels from the download client.
+   * - qBittorrent: Returns configured category names
+   * - Transmission: Returns empty array (uses free-form labels)
+   * - Usenet clients: Returns empty array (feature scoped to torrent clients)
+   */
+  getCategories(): Promise<string[]>;
+
+  /**
+   * Set the category/label for a download.
+   * - qBittorrent: Sets torrent category
+   * - Transmission: Sets torrent label
+   * - Usenet clients: No-op
+   * @param id - Download ID
+   * @param category - Category/label name to assign
+   */
+  setCategory(id: string, category: string): Promise<void>;
 }