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/components/admin/download-clients/DownloadClientCard.tsx

@@ -16,6 +16,7 @@ interface DownloadClientCardProps {
     url: string;
     enabled: boolean;
     customPath?: string;
+    postImportCategory?: string;
   };
   onEdit: () => void;
   onDelete: () => void;
@@ -62,6 +63,11 @@ export function DownloadClientCard({ client, onEdit, onDelete }: DownloadClientC
                 Path: {client.customPath}
               </p>
             )}
+            {client.postImportCategory && (
+              <p className="text-xs text-purple-600 dark:text-purple-400 truncate" title={`Post-import category: ${client.postImportCategory}`}>
+                Post-import: {client.postImportCategory}
+              </p>
+            )}
           </div>
         </div>