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/lib/services/auth/LocalAuthProvider.ts

@@ -150,7 +150,11 @@ export class LocalAuthProvider implements IAuthProvider {
         return { success: false, error: 'Username must be at least 3 characters' };
       }
 
-      if (!password || password.length < 8) {
+      const allowWeakPassword = process.env.ALLOW_WEAK_PASSWORD === 'true';
+      if (!password) {
+        return { success: false, error: 'Password is required' };
+      }
+      if (!allowWeakPassword && password.length < 8) {
         return { success: false, error: 'Password must be at least 8 characters' };
       }