[no-issue]; fix: MultipleBagFetchException — @EntityGraph 제거, @batchsize 적용

두 개의 @onetomany List 컬렉션을 @EntityGraph로 동시에 JOIN FETCH하면
Hibernate MultipleBagFetchException이 발생한다.
@EntityGraph를 제거하고 @batchsize(size=100)으로 교체하여
lazy 로딩 + IN절 배치 쿼리 방식으로 N+1 없이 해결.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: leeminseo0923

src/main/kotlin/com/yourssu/scouter/common/storage/domain/mail/JpaMailRepository.kt

@@ -1,7 +1,6 @@
 package com.yourssu.scouter.common.storage.domain.mail
 
 import com.yourssu.scouter.common.implement.domain.mail.reservation.MailReservationStatus
-import org.springframework.data.jpa.repository.EntityGraph
 import org.springframework.data.jpa.repository.JpaRepository
 import org.springframework.data.jpa.repository.Modifying
 import org.springframework.data.jpa.repository.Query
@@ -10,56 +9,44 @@ import java.time.Instant
 import java.util.Optional
 
 interface JpaMailRepository : JpaRepository<MailEntity, Long> {
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     override fun findById(id: Long): Optional<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     override fun findAll(): List<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     fun findAllByGroupId(groupId: Long): List<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     fun findAllByReservationTimeLessThanEqual(reservationTime: Instant): List<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     fun findAllByReservationTimeLessThanEqualAndStatusNot(
         reservationTime: Instant,
         status: MailReservationStatus,
     ): List<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     fun findAllByReservationTimeLessThanEqualAndStatusIn(
         reservationTime: Instant,
         statuses: Collection<MailReservationStatus>,
     ): List<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     fun findAllBySenderEmailAddress(senderEmailAddress: String): List<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     fun findAllBySenderEmailAddressIn(senderEmailAddresses: List<String>): List<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     fun findAllByReservationTimeLessThanEqualAndSenderEmailAddress(
         reservationTime: Instant,
         senderEmailAddress: String,
     ): List<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     fun findAllByReservationTimeLessThanEqualAndSenderEmailAddressIn(
         reservationTime: Instant,
         senderEmailAddresses: List<String>,
     ): List<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     fun findAllBySenderEmailAddressAndReservationTimeBetween(
         senderEmailAddress: String,
         from: Instant,
         to: Instant,
     ): List<MailEntity>
 
-    @EntityGraph(attributePaths = ["recipientEmailAddress", "attachments"])
     fun findAllBySenderEmailAddressInAndReservationTimeBetween(
         senderEmailAddresses: List<String>,
         from: Instant,

src/main/kotlin/com/yourssu/scouter/common/storage/domain/mail/MailEntity.kt

@@ -17,6 +17,7 @@ import jakarta.persistence.Lob
 import jakarta.persistence.OneToMany
 import jakarta.persistence.Table
 import jakarta.persistence.Transient
+import org.hibernate.annotations.BatchSize
 import org.hibernate.annotations.JdbcTypeCode
 import org.hibernate.type.SqlTypes
 import java.time.Instant
@@ -46,8 +47,10 @@ class MailEntity(
     val status: MailReservationStatus = MailReservationStatus.SCHEDULED,
     @Column
     val claimedAt: Instant? = null,
+    @BatchSize(size = 100)
     @OneToMany(mappedBy = "mail", fetch = FetchType.LAZY, cascade = [CascadeType.ALL], orphanRemoval = true)
     val recipientEmailAddress: MutableList<MailRecipientAddressEntity> = mutableListOf(),
+    @BatchSize(size = 100)
     @OneToMany(mappedBy = "mail", fetch = FetchType.LAZY, cascade = [CascadeType.ALL], orphanRemoval = true)
     val attachments: MutableList<MailAttachmentEntity> = mutableListOf(),
 ) {