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/PathsStep.tsx

@@ -14,7 +14,8 @@ interface PathsStepProps {
   mediaDir: string;
   metadataTaggingEnabled: boolean;
   chapterMergingEnabled: boolean;
-  onUpdate: (field: string, value: string | boolean) => void;
+  pathsTested: boolean;
+  onUpdate: (field: string, value: any) => void;
   onNext: () => void;
   onBack: () => void;
 }
@@ -24,6 +25,7 @@ export function PathsStep({
   mediaDir,
   metadataTaggingEnabled,
   chapterMergingEnabled,
+  pathsTested,
   onUpdate,
   onNext,
   onBack,
@@ -34,7 +36,11 @@ export function PathsStep({
     message: string;
     downloadDirValid?: boolean;
     mediaDirValid?: boolean;
-  } | null>(null);
+  } | null>(
+    pathsTested
+      ? { success: true, message: 'Paths validated previously.', downloadDirValid: true, mediaDirValid: true }
+      : null
+  );
 
   const testPaths = async () => {
     setTesting(true);
@@ -59,6 +65,7 @@ export function PathsStep({
           downloadDirValid: data.downloadDirValid,
           mediaDirValid: data.mediaDirValid,
         });
+        onUpdate('pathsTested', true);
       } else {
         setTestResult({
           success: false,
@@ -66,12 +73,14 @@ export function PathsStep({
           downloadDirValid: data.downloadDirValid,
           mediaDirValid: data.mediaDirValid,
         });
+        onUpdate('pathsTested', false);
       }
     } catch (error) {
       setTestResult({
         success: false,
         message: error instanceof Error ? error.message : 'Path validation failed',
       });
+      onUpdate('pathsTested', false);
     } finally {
       setTesting(false);
     }