feat(home): AI 캐릭터 스냅샷 집계 기간을 정의한다

src/test/kotlin/kr/co/vividnext/sodalive/v2/recommendation/domain/RecommendationSnapshotWindowPolicyTest.kt

@@ -0,0 +1,20 @@
+package kr.co.vividnext.sodalive.v2.recommendation.domain
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.DisplayName
+import org.junit.jupiter.api.Test
+import java.time.LocalDateTime
+
+class RecommendationSnapshotWindowPolicyTest {
+    private val policy = RecommendationSnapshotWindowPolicy()
+
+    @Test
+    @DisplayName("KST 전날 하루를 UTC 조회 범위로 변환한다")
+    fun shouldConvertPreviousKstDayToUtcWindow() {
+        val window = policy.previousKstDayUtcWindow(LocalDateTime.of(2026, 7, 9, 21, 0, 0))
+
+        assertEquals(LocalDateTime.of(2026, 7, 8, 15, 0, 0), window.startUtc)
+        assertEquals(LocalDateTime.of(2026, 7, 9, 15, 0, 0), window.endExclusiveUtc)
+        assertEquals(LocalDateTime.of(2026, 7, 9, 14, 59, 59), window.snapshotAt)
+    }
+}