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.

tests/app/setup/steps/AudiobookshelfStep.test.tsx

@@ -24,6 +24,7 @@ const AudiobookshelfHarness = ({
     absApiToken: 'token',
     absLibraryId: '',
     absTriggerScanAfterImport: false,
+    absLibraries: [] as { id: string; name: string; itemCount: number }[],
     ...initialState,
   });
 

tests/app/setup/steps/PlexStep.test.tsx

@@ -24,6 +24,7 @@ const PlexHarness = ({
     plexToken: 'token',
     plexLibraryId: '',
     plexTriggerScanAfterImport: false,
+    plexLibraries: [] as { id: string; title: string; type: string }[],
     ...initialState,
   });