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/notifications/providers/route.ts

@@ -0,0 +1,42 @@
+/**
+ * Component: Notification Providers Metadata API
+ * Documentation: documentation/backend/services/notifications.md
+ */
+
+import { NextRequest, NextResponse } from 'next/server';
+import { requireAuth, requireAdmin, AuthenticatedRequest } from '@/lib/middleware/auth';
+import { getAllProviderMetadata } from '@/lib/services/notification';
+import { RMABLogger } from '@/lib/utils/logger';
+
+const logger = RMABLogger.create('API.Admin.Notifications.Providers');
+
+/**
+ * GET /api/admin/notifications/providers
+ * Returns metadata for all registered notification providers
+ */
+export async function GET(request: NextRequest) {
+  return requireAuth(request, async (req: AuthenticatedRequest) => {
+    return requireAdmin(req, async () => {
+      try {
+        const providers = getAllProviderMetadata();
+
+        return NextResponse.json({
+          success: true,
+          providers,
+        });
+      } catch (error) {
+        logger.error('Failed to fetch provider metadata', {
+          error: error instanceof Error ? error.message : String(error),
+        });
+
+        return NextResponse.json(
+          {
+            error: 'FetchError',
+            message: 'Failed to fetch provider metadata',
+          },
+          { status: 500 }
+        );
+      }
+    });
+  });
+}