feat(home): AI 캐릭터 스냅샷 집계 기간을 정의한다

src/main/kotlin/kr/co/vividnext/sodalive/v2/recommendation/domain/RecommendationSnapshotWindowPolicy.kt

@@ -0,0 +1,41 @@
+package kr.co.vividnext.sodalive.v2.recommendation.domain
+
+import java.time.LocalDateTime
+import java.time.ZoneId
+
+class RecommendationSnapshotWindowPolicy {
+    fun previousKstDayUtcWindow(nowUtc: LocalDateTime): RecommendationSnapshotWindow {
+        val previousKstDate = nowUtc
+            .atZone(UTC_ZONE)
+            .withZoneSameInstant(KST_ZONE)
+            .toLocalDate()
+            .minusDays(1)
+        val startUtc = previousKstDate
+            .atStartOfDay(KST_ZONE)
+            .withZoneSameInstant(UTC_ZONE)
+            .toLocalDateTime()
+        val endExclusiveUtc = previousKstDate
+            .plusDays(1)
+            .atStartOfDay(KST_ZONE)
+            .withZoneSameInstant(UTC_ZONE)
+            .toLocalDateTime()
+        val snapshotAt = endExclusiveUtc.minusSeconds(1)
+
+        return RecommendationSnapshotWindow(
+            startUtc = startUtc,
+            endExclusiveUtc = endExclusiveUtc,
+            snapshotAt = snapshotAt
+        )
+    }
+
+    companion object {
+        private val UTC_ZONE: ZoneId = ZoneId.of("UTC")
+        private val KST_ZONE: ZoneId = ZoneId.of("Asia/Seoul")
+    }
+}
+
+data class RecommendationSnapshotWindow(
+    val startUtc: LocalDateTime,
+    val endExclusiveUtc: LocalDateTime,
+    val snapshotAt: LocalDateTime
+)

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

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+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)
+    }
+}