autoresearch iter 16: add batch user lookup and use it for log enrichment

enrichAccessLogsWithUsers preferred the UserService contract over the local
repository — correct layering, but it looped GetUserByID and issued up to a
page-size worth of separate SELECTs against w_users, while the single-query
WHERE id IN variant was only reached in the no-contract fallback branch.
Give the contract a GetUsersByIDs so callers can keep the layering and drop
the N+1. The test asserts 1 query batched against 3 per-id, so the counting
itself is checked.

backend/plugins/domain/user/service.go

@@ -62,6 +62,18 @@ func (s *userServiceImpl) GetUserByID(ctx context.Context, id uint64) (*contract
 	return toUserDTO(u), nil
 }
 
+func (s *userServiceImpl) GetUsersByIDs(ctx context.Context, ids []uint64) ([]*contracts.UserDTO, error) {
+	users, err := GetUsersByIDs(ctx, ids)
+	if err != nil {
+		return nil, err
+	}
+	dtos := make([]*contracts.UserDTO, 0, len(users))
+	for i := range users {
+		dtos = append(dtos, toUserDTO(&users[i]))
+	}
+	return dtos, nil
+}
+
 func (s *userServiceImpl) GetUserByUsername(ctx context.Context, username string) (*contracts.UserDTO, error) {
 	u, err := GetUserByUsername(ctx, username)
 	if err != nil {