feat(ranking): 스냅샷 완전 공백 조회를 추가한다

src/main/kotlin/kr/co/vividnext/sodalive/v2/ranking/adapter/out/persistence/DefaultCreatorRankingSnapshotRepository.kt

@@ -28,6 +28,10 @@ class DefaultCreatorRankingSnapshotRepository(
         return repository.findPreviousCompletedSnapshots().map { it.toRecord() }
     }
 
+    override fun isSnapshotTableEmpty(): Boolean {
+        return repository.count() == 0L
+    }
+
     @Transactional
     override fun replaceSnapshots(
         aggregationStartAtUtc: LocalDateTime,

src/main/kotlin/kr/co/vividnext/sodalive/v2/ranking/port/out/CreatorRankingSnapshotPort.kt

@@ -12,6 +12,8 @@ interface CreatorRankingSnapshotPort {
 
     fun findPreviousCompletedSnapshots(): List<CreatorRankingSnapshotRecord>
 
+    fun isSnapshotTableEmpty(): Boolean
+
     fun replaceSnapshots(
         aggregationStartAtUtc: LocalDateTime,
         aggregationEndAtUtc: LocalDateTime,

src/test/kotlin/kr/co/vividnext/sodalive/v2/ranking/adapter/out/persistence/DefaultCreatorRankingSnapshotRepositoryTest.kt

@@ -147,6 +147,22 @@ class DefaultCreatorRankingSnapshotRepositoryTest @Autowired constructor(
         assertEquals(emptyList<CreatorRankingSnapshotRecord>(), snapshots)
     }
 
+    @Test
+    @DisplayName("스냅샷 row가 하나도 없을 때만 테이블 완전 공백으로 판단한다")
+    fun shouldReturnTrueOnlyWhenSnapshotTableHasNoRows() {
+        assertEquals(true, adapter.isSnapshotTableEmpty())
+
+        repository.save(
+            snapshot(
+                creatorId = 1L,
+                aggregationStartAtUtc = LocalDateTime.of(2026, 5, 24, 15, 0),
+                aggregationEndAtUtc = LocalDateTime.of(2026, 5, 31, 15, 0)
+            )
+        )
+
+        assertEquals(false, adapter.isSnapshotTableEmpty())
+    }
+
     @Test
     @DisplayName("20위 점수 경계 동점 후보는 저장소에서 누락 없이 저장하고 조회할 수 있다")
     fun shouldPersistAllCandidatesTiedAtTwentiethScoreBoundary() {

src/test/kotlin/kr/co/vividnext/sodalive/v2/ranking/application/CreatorRankingSnapshotRefreshServiceTest.kt

@@ -256,6 +256,8 @@ private class FakeCreatorRankingSnapshotPort : CreatorRankingSnapshotPort {
 
     override fun findPreviousCompletedSnapshots(): List<CreatorRankingSnapshotRecord> = snapshots
 
+    override fun isSnapshotTableEmpty(): Boolean = snapshots.isEmpty()
+
     override fun replaceSnapshots(
         aggregationStartAtUtc: LocalDateTime,
         aggregationEndAtUtc: LocalDateTime,