feat: add support for Pushover notification provider

- Updated the list of supported notification provider types to include 'pushover'.
- Enhanced the notifications API tests to validate Pushover integration.
- Modified the notifications form to include fields specific to Pushover, such as API Token and User Key.
- Implemented CRUD operations for Pushover providers in the settings.
- Added end-to-end tests for Pushover provider functionality, including form rendering, payload validation, and security checks.
- Updated translations to include Pushover-specific labels and placeholders.

frontend/src/components/__tests__/SecurityNotificationSettingsModal.test.tsx

@@ -86,7 +86,7 @@ describe('Security Notification Settings on Notifications page', () => {
     await user.click(await screen.findByTestId('add-provider-btn'));
 
     const typeSelect = screen.getByTestId('provider-type') as HTMLSelectElement;
-    expect(Array.from(typeSelect.options).map((option) => option.value)).toEqual(['discord', 'gotify', 'webhook', 'email', 'telegram', 'slack']);
+    expect(Array.from(typeSelect.options).map((option) => option.value)).toEqual(['discord', 'gotify', 'webhook', 'email', 'telegram', 'slack', 'pushover']);
     expect(typeSelect.value).toBe('discord');
 
     const webhookInput = screen.getByTestId('provider-url') as HTMLInputElement;