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/login/page.tsx

@@ -38,6 +38,7 @@ function LoginContent() {
     hasLocalUsers: boolean;
     oidcProviderName: string | null;
     localLoginDisabled: boolean;
+    allowWeakPassword: boolean;
     automationEnabled: boolean;
   } | null>(null);
   const [showRegisterForm, setShowRegisterForm] = useState(false);
@@ -78,6 +79,7 @@ function LoginContent() {
           hasLocalUsers: false,
           oidcProviderName: null,
           localLoginDisabled: false,
+          allowWeakPassword: false,
           automationEnabled: false,
         });
       }
@@ -345,7 +347,7 @@ function LoginContent() {
       return;
     }
 
-    if (registerPassword.length < 8) {
+    if (!authProviders?.allowWeakPassword && registerPassword.length < 8) {
       setError('Password must be at least 8 characters');
       setIsLoggingIn(false);
       return;
@@ -639,10 +641,12 @@ function LoginContent() {
                             className="w-full px-4 py-3 bg-gray-800 border border-gray-700 rounded-lg text-white placeholder-gray-500 focus:outline-none focus:ring-2 focus:ring-orange-500 focus:border-transparent"
                             placeholder="••••••••"
                             required
-                            minLength={8}
+                            minLength={authProviders?.allowWeakPassword ? 1 : 8}
                             autoComplete="new-password"
                           />
-                          <p className="text-xs text-gray-500 mt-1">At least 8 characters</p>
+                          {!authProviders?.allowWeakPassword && (
+                            <p className="text-xs text-gray-500 mt-1">At least 8 characters</p>
+                          )}
                         </div>
                         <div>
                           <label htmlFor="register-confirm-password" className="block text-sm font-medium text-gray-300 mb-2">
@@ -656,7 +660,7 @@ function LoginContent() {
                             className="w-full px-4 py-3 bg-gray-800 border border-gray-700 rounded-lg text-white placeholder-gray-500 focus:outline-none focus:ring-2 focus:ring-orange-500 focus:border-transparent"
                             placeholder="••••••••"
                             required
-                            minLength={8}
+                            minLength={authProviders?.allowWeakPassword ? 1 : 8}
                             autoComplete="new-password"
                           />
                         </div>