Integrate EmailService to invitation requests

[serggalel]

As an Organizer I want to send the inviting request emails to students so that they can be notified instantly about the invitation.

Preconditions

• The controller's endpoint and service's method to send invitations already exist with necessary checks.

Acceptance Criteria

1. When the sendEnrollmentRequest method successfully executes (the invitation record is created with PENDING status), an email is dispatched to the user's registered email address inviting them to participate in the specified competition/stage.
2. The email sending process happens asynchronously. The ORG's HTTP request to send the invitation must return a 201 Created immediately, without waiting for the email server to respond.
3. If the email service is temporarily down or times out, it does not roll back the database transaction. The invitation remains in the PENDING status, and the email failure is gracefully logged (or sent to a retry queue).

Epic link
#5