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/setup/steps/PlexStep.tsx

@@ -14,7 +14,8 @@ interface PlexStepProps {
   plexToken: string;
   plexLibraryId: string;
   plexTriggerScanAfterImport: boolean;
-  onUpdate: (field: string, value: string | boolean) => void;
+  plexLibraries: PlexLibrary[];
+  onUpdate: (field: string, value: any) => void;
   onNext: () => void;
   onBack: () => void;
 }
@@ -30,6 +31,7 @@ export function PlexStep({
   plexToken,
   plexLibraryId,
   plexTriggerScanAfterImport,
+  plexLibraries,
   onUpdate,
   onNext,
   onBack,
@@ -39,8 +41,12 @@ export function PlexStep({
     success: boolean;
     message: string;
     libraries?: PlexLibrary[];
-  } | null>(null);
-  const [libraries, setLibraries] = useState<PlexLibrary[]>([]);
+  } | null>(
+    plexLibraries.length > 0
+      ? { success: true, message: 'Connection verified previously.' }
+      : null
+  );
+  const [libraries, setLibraries] = useState<PlexLibrary[]>(plexLibraries);
 
   const testConnection = async () => {
     setTesting(true);
@@ -56,12 +62,14 @@ export function PlexStep({
       const data = await response.json();
 
       if (response.ok && data.success) {
+        const libs = data.libraries || [];
         setTestResult({
           success: true,
           message: `Connected to ${data.serverName || 'Plex server'} successfully!`,
-          libraries: data.libraries || [],
+          libraries: libs,
         });
-        setLibraries(data.libraries || []);
+        setLibraries(libs);
+        onUpdate('plexLibraries', libs);
       } else {
         setTestResult({
           success: false,