fix(server): return slug when listing all trainings

internal/server/api.go

@@ -50,6 +50,7 @@ func (h *APIHandlers) ListTrainings(ctx context.Context, req ListTrainingsReques
 		data[idx] = Training{
 			Id:          t.ID,
 			Name:        t.Name,
+			Slug:        t.Slug,
 			Days:        t.Days,
 			Description: t.Description,
 			Pricing:     pricing,