feat(projects): add Intern members atomically

src/main/java/com/lab/labtimesheet/feature/project/service/ProjectService.java

@@ -1,6 +1,7 @@
 package com.lab.labtimesheet.feature.project.service;
 
 import com.lab.labtimesheet.feature.project.exception.ProjectAccessDeniedException;
+import com.lab.labtimesheet.feature.project.exception.ProjectRuleViolationException;
 import com.lab.labtimesheet.feature.account.service.AccountService;
 import com.lab.labtimesheet.feature.project.model.ProjectInternEligibility;
 import com.lab.labtimesheet.feature.project.model.dto.ProjectCreateCommand;
@@ -9,6 +10,9 @@ import com.lab.labtimesheet.feature.project.model.entity.ProjectEntity;
 import com.lab.labtimesheet.feature.project.repository.ProjectRepository;
 import com.lab.labtimesheet.feature.task.service.TaskQueryService;
 import java.time.Clock;
+import java.time.LocalDate;
+import java.util.HashSet;
+import java.util.List;
 import java.util.Set;
 import java.util.stream.Collectors;
 import lombok.RequiredArgsConstructor;
@@ -68,9 +72,42 @@ public class ProjectService {
      */
     @Transactional
     public void addMember(long actorUserId, long projectId, long internUserId) {
+        addMembers(actorUserId, projectId, List.of(internUserId));
+    }
+
+    /**
+     * Adds a complete selection of eligible nonmembers while holding one Project write lock.
+     * Every identifier is revalidated after owner authorization and before the aggregate changes,
+     * so missing, duplicate, stale, ineligible, or current-member selections leave membership
+     * unchanged.
+     *
+     * @param actorUserId authenticated owning Mentor
+     * @param projectId Project to update
+     * @param internUserIds distinct eligible Intern account identifiers
+     * @throws com.lab.labtimesheet.feature.project.exception.ProjectRuleViolationException when
+     *         the selection is null, empty, malformed, duplicate, stale, ineligible, or already
+     *         contains a current member
+     */
+    @Transactional
+    public void addMembers(long actorUserId, long projectId, List<Long> internUserIds) {
         var project = lockedProject(projectId);
         project.authorizeOwner(actorUserId);
-        project.addMember(actorUserId, eligibleIntern(internUserId), clock.instant());
+        if (internUserIds == null || internUserIds.isEmpty()) {
+            throw new ProjectRuleViolationException("Select at least one Intern");
+        }
+        if (internUserIds.stream().anyMatch(userId -> userId == null || userId <= 0)
+                || new HashSet<>(internUserIds).size() != internUserIds.size()) {
+            throw new ProjectRuleViolationException("Intern selection is invalid");
+        }
+
+        var selectedInterns = internUserIds.stream().map(this::eligibleIntern).toList();
+        if (selectedInterns.stream().anyMatch(intern -> !intern.isEligible())
+                || selectedInterns.stream().anyMatch(intern -> project.hasCurrentMember(intern.userId()))) {
+            throw new ProjectRuleViolationException("One or more selected Interns are no longer eligible");
+        }
+
+        var addedAt = clock.instant();
+        selectedInterns.forEach(intern -> project.addMember(actorUserId, intern, addedAt));
         projects.flush();
     }
 
@@ -145,7 +182,7 @@ public class ProjectService {
     }
 
     private ProjectInternEligibility eligibleIntern(long userId) {
-        return new ProjectInternEligibility(userId, accounts.isEligibleIntern(userId));
+        return new ProjectInternEligibility(userId, accounts.isEligibleIntern(userId, LocalDate.now(clock)));
     }
 
     private void requireActiveMentor(long userId) {