feat: add trip invitation creation and acceptance flow

backend/libs/trips/src/trips.module.ts

@@ -7,6 +7,8 @@ import { TripSettingsService } from './trip-settings.service';
 import { TripMembersRepository } from './trip-members.repository';
 import { TripMembersService } from './trip-members.service';
 import { TripMembershipGuard } from './trip-membership.guard';
+import { TripInvitationsRepository } from './trip-invitations.repository';
+import { TripInvitationsService } from './trip-invitations.service';
 
 @Module({
   imports: [DatabaseModule],
@@ -18,12 +20,15 @@ import { TripMembershipGuard } from './trip-membership.guard';
     TripMembersRepository,
     TripMembersService,
     TripMembershipGuard,
+    TripInvitationsRepository,
+    TripInvitationsService,
   ],
   exports: [
     TripsService,
     TripSettingsService,
     TripMembersService,
     TripMembershipGuard,
+    TripInvitationsService,
   ],
 })
 export class TripsLibModule {}