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

src/app/api/admin/settings/download-clients/categories/route.ts

@@ -0,0 +1,104 @@
+/**
+ * Component: Fetch Download Client Categories API
+ * Documentation: documentation/phase3/download-clients.md
+ */
+
+import { NextRequest, NextResponse } from 'next/server';
+import { requireAuth, requireAdmin, AuthenticatedRequest } from '@/lib/middleware/auth';
+import { getConfigService } from '@/lib/services/config.service';
+import { getDownloadClientManager, DownloadClientConfig } from '@/lib/services/download-client-manager.service';
+import { SUPPORTED_CLIENT_TYPES } from '@/lib/interfaces/download-client.interface';
+import { RMABLogger } from '@/lib/utils/logger';
+
+const logger = RMABLogger.create('API.Admin.Settings.DownloadClients.Categories');
+
+/**
+ * POST - Fetch categories from a download client
+ * Accepts same connection config as the test endpoint
+ */
+export async function POST(request: NextRequest) {
+  return requireAuth(request, async (req: AuthenticatedRequest) => {
+    return requireAdmin(req, async () => {
+      try {
+        const body = await request.json();
+        const {
+          clientId,
+          type,
+          name: clientName,
+          url,
+          username,
+          password,
+          disableSSLVerify,
+          remotePathMappingEnabled,
+          remotePath,
+          localPath,
+        } = body;
+
+        if (!SUPPORTED_CLIENT_TYPES.includes(type)) {
+          return NextResponse.json(
+            { error: `Invalid client type. Must be one of: ${SUPPORTED_CLIENT_TYPES.join(', ')}` },
+            { status: 400 }
+          );
+        }
+
+        if (!url) {
+          return NextResponse.json(
+            { error: 'URL is required' },
+            { status: 400 }
+          );
+        }
+
+        const config = await getConfigService();
+        const manager = getDownloadClientManager(config);
+
+        // If editing and password not provided, use stored password
+        let effectivePassword = password;
+        let effectiveUsername = username;
+
+        if (clientId && !password) {
+          const existingClients = await manager.getAllClients();
+          const existingClient = existingClients.find(c => c.id === clientId);
+
+          if (!existingClient) {
+            return NextResponse.json(
+              { error: 'Client not found' },
+              { status: 404 }
+            );
+          }
+
+          effectivePassword = existingClient.password;
+          if (!username && existingClient.username) {
+            effectiveUsername = existingClient.username;
+          }
+        }
+
+        const testConfig: DownloadClientConfig = {
+          id: 'categories-fetch',
+          type,
+          name: clientName || type,
+          enabled: true,
+          url,
+          username: effectiveUsername || '',
+          password: effectivePassword || '',
+          disableSSLVerify: disableSSLVerify || false,
+          remotePathMappingEnabled: remotePathMappingEnabled || false,
+          remotePath: remotePath || undefined,
+          localPath: localPath || undefined,
+          category: 'readmeabook',
+        };
+
+        const service = await manager.createClientFromConfig(testConfig);
+        const categories = await service.getCategories();
+
+        return NextResponse.json({ success: true, categories });
+      } catch (error) {
+        const message = error instanceof Error ? error.message : String(error);
+        logger.error('Failed to fetch categories', { error: message });
+        return NextResponse.json(
+          { success: false, error: message },
+          { status: 400 }
+        );
+      }
+    });
+  });
+}

src/app/api/admin/settings/download-clients/route.ts

@@ -63,6 +63,7 @@ export async function POST(request: NextRequest) {
           localPath,
           category,
           customPath,
+          postImportCategory,
         } = body;
 
         // Validate type
@@ -138,6 +139,7 @@ export async function POST(request: NextRequest) {
           localPath: localPath || undefined,
           category: category || 'readmeabook',
           customPath: customPath || undefined,
+          postImportCategory: postImportCategory || undefined,
         };
 
         // Test connection before saving