Use domain and API key from app settings in SendEmailViaMailgun

[selfmadecode]

The SendEmailViaMailgun method in MailService should retrieve the Mailgun domain and API key from the application settings instead of using hardcoded values.

Steps to fix:

Open the MailService class.

Locate the SendEmailViaMailgun method.

Update the method to read the domain and API key from the application configuration (e.g., appsettings.json or environment variables).

Expected result:
The method uses configuration values for the Mailgun domain and API key, improving maintainability and security.

[selfmadecode]

We can also mark the method as static