feat(home-following): 최근 소식 source key를 추가한다

src/main/kotlin/kr/co/vividnext/sodalive/v2/home/following/domain/HomeFollowingNewsSourceKey.kt

@@ -0,0 +1,17 @@
+package kr.co.vividnext.sodalive.v2.home.following.domain
+
+import java.time.LocalDateTime
+
+object HomeFollowingNewsSourceKey {
+    fun creatorRanking(creatorId: Long, aggregationStartAtUtc: LocalDateTime): String {
+        return "${FollowingNewsType.CREATOR_RANKING.name}:$creatorId:$aggregationStartAtUtc"
+    }
+
+    fun audioContent(contentId: Long): String {
+        return "${FollowingNewsType.AUDIO_CONTENT.name}:$contentId"
+    }
+
+    fun communityPost(postId: Long): String {
+        return "${FollowingNewsType.COMMUNITY_POST.name}:$postId"
+    }
+}

src/test/kotlin/kr/co/vividnext/sodalive/v2/home/following/domain/HomeFollowingNewsSourceKeyTest.kt

@@ -0,0 +1,33 @@
+package kr.co.vividnext.sodalive.v2.home.following.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 HomeFollowingNewsSourceKeyTest {
+    @Test
+    @DisplayName("크리에이터 랭킹 source key는 타입, 크리에이터, 집계 시작 시각으로 생성한다")
+    fun shouldCreateCreatorRankingSourceKey() {
+        val aggregationStartAtUtc = LocalDateTime.of(2026, 5, 31, 15, 0)
+
+        val sourceKey = HomeFollowingNewsSourceKey.creatorRanking(
+            creatorId = 10L,
+            aggregationStartAtUtc = aggregationStartAtUtc
+        )
+
+        assertEquals("CREATOR_RANKING:10:2026-05-31T15:00", sourceKey)
+    }
+
+    @Test
+    @DisplayName("오디오 콘텐츠 source key는 타입과 콘텐츠 id로 생성한다")
+    fun shouldCreateAudioContentSourceKey() {
+        assertEquals("AUDIO_CONTENT:300", HomeFollowingNewsSourceKey.audioContent(contentId = 300L))
+    }
+
+    @Test
+    @DisplayName("커뮤니티 게시글 source key는 타입과 게시글 id로 생성한다")
+    fun shouldCreateCommunityPostSourceKey() {
+        assertEquals("COMMUNITY_POST:400", HomeFollowingNewsSourceKey.communityPost(postId = 400L))
+    }
+}