Configures the system-level Google OAuth 2.0 credentials that allow the platform to read Gmail mailboxes on behalf of users (for the inbox integration feature).

###### Creating OAuth 2.0 Credentials

1. Open the Google Cloud Console and enable the Gmail API for your project.
2. Go to APIs & Services → Credentials → Create Credentials → OAuth client ID.
3. Select Web application as the application type.
4. Add an authorised redirect URI: `https://<your-domain>/app/api/gmail/callback`
5. Copy the Client ID and Client Secret.
6. Configure the OAuth consent screen and add the scope `https://www.googleapis.com/auth/gmail.readonly` (and any additional scopes needed).

###### Settings

Client ID
  The OAuth 2.0 client ID from Google Cloud Console.
Client Secret
  The OAuth 2.0 client secret. Stored encrypted at rest.
Redirect Domain
  The domain used in the redirect URI registered with Google. This is required to match the domain used in the authorized redirect URI configured in the Google Cloud Console (e.g. `https://<your-domain>/app/api/gmail/callback` would require a redirect domain of `<your-domain>`. ).

Open Google Cloud Credentials