Implement getting pending invitations

[serggalel]

As an Organizer I want to view a list of invitations for a specific competition stage so that I can track their statuses .

Assumptions & Constraints
The number of invitations per stage could be large, making pagination mandatory to prevent database and frontend overload.

Preconditions

1. The invitation flow is already created and organizers can successfully submit invitations and students can accept or reject them.
2. The requester is authenticated and holds the ORG role.

Business rules
Data Completeness: The list must provide enough context (student's email and first name and surname, status etc) so the frontend can render a table without needing to make a separate API call for every single user.

Acceptance Criteria

1. When an Organizer requests the list of invitations, the system returns an HTTP 200 OK.
2. Search filters students by email, first name and surname, case-insensitively.
3. The endpoint supports pagination via page and size query parameters. By default, the list is sorted by submittedAt in descending order (newest first).

Epic link
#5