feat: add Slack notification provider support

- Updated the notification provider types to include 'slack'.
- Modified API tests to handle 'slack' as a valid provider type.
- Enhanced frontend forms to display Slack-specific fields (webhook URL and channel name).
- Implemented CRUD operations for Slack providers, ensuring proper payload structure.
- Added E2E tests for Slack notification provider, covering form rendering, validation, and security checks.
- Updated translations to include Slack-related text.
- Ensured that sensitive information (like tokens) is not exposed in API responses.

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: 'slack' })).rejects.toThrow('Unsupported notification provider type: slack')
+    await expect(createProvider({ name: 'x', type: 'pushover' })).rejects.toThrow('Unsupported notification provider type: pushover')
     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' })