Some Backend files have no unit test coverage

[imnasnainaec]

• Backend/Controllers/InviteController.cs ([InviteController] Clean up logic and add tests #2663)
• Backend/Controllers/StatisticsController.cs ([StatisticsController] Add tests #2669)
• Backend/Services/EmailService.cs ([EmailService] Exclude from test coverage #2671)
• Backend/Services/InviteService.cs ([InviteService] Add tests #2672)