fix: mailgun 401 error

[RMCampos]

This pull request refactors the email service integration for Mailgun in the backend, replacing the legacy RestTemplate-based implementation with the newer RestClient API. It also updates the related tests and environment variable handling to align with these changes.

Mailgun Email Service Refactor:

• Migrated MailgunEmailService from using RestTemplate to the modern RestClient API, simplifying HTTP request handling and improving error logging. The constructor now takes a RestClient.Builder and sets up base URL, authentication, and error handling directly. [1] [2]
• Refactored the sendEmail method to use restClient's fluent API for sending emails, removing manual header and entity construction, and improving error handling for both HTTP and unexpected exceptions. [1] [2]

Testing Updates:

• Updated MailgunEmailServiceTest to mock RestClient and its builder, replacing all RestTemplate usage and verifications. Introduced a helper method to set up the mock call chain for the new client. [1] [2] [3] [4] [5]

Dependency and Environment Variable Adjustments:

• Removed the unused spring-boot-starter-mail dependency from pom.xml since Mailgun is now handled directly via HTTP.
• Updated the Docker startup script to pass MAILGUN_APIKEY and SECURITY_KEY as environment variables for the server container.