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/app/api/admin/settings/download-clients/[id]/route.ts

@@ -38,6 +38,7 @@ export async function PUT(
           localPath,
           category,
           customPath,
+          postImportCategory,
         } = body;
 
         const config = await getConfigService();
@@ -76,6 +77,7 @@ export async function PUT(
           localPath: localPath !== undefined ? localPath : existingClient.localPath,
           category: category !== undefined ? category : existingClient.category,
           customPath: customPath !== undefined ? (customPath || undefined) : existingClient.customPath,
+          postImportCategory: postImportCategory !== undefined ? (postImportCategory || undefined) : existingClient.postImportCategory,
         };
 
         // Validate path mapping if enabled