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/api/__tests__/notifications.test.ts

@@ -53,7 +53,7 @@ describe('notifications api', () => {
     await testProvider({ id: '2', name: 'test', type: 'discord' })
     expect(client.post).toHaveBeenCalledWith('/notifications/providers/test', { id: '2', name: 'test', type: 'discord' })
 
-    await expect(createProvider({ name: 'x', type: 'pushover' })).rejects.toThrow('Unsupported notification provider type: pushover')
+    await expect(createProvider({ name: 'x', type: 'pagerduty' })).rejects.toThrow('Unsupported notification provider type: pagerduty')
     await expect(updateProvider('2', { name: 'updated', type: 'generic' })).rejects.toThrow('Unsupported notification provider type: generic')
     await testProvider({ id: '2', name: 'test', type: 'telegram' })
     expect(client.post).toHaveBeenCalledWith('/notifications/providers/test', { id: '2', name: 'test', type: 'telegram' })