feat(home): 인기 커뮤니티 fallback을 홈 조회에 연결한다

src/main/kotlin/kr/co/vividnext/sodalive/v2/recommendation/application/HomeRecommendationQueryService.kt

@@ -27,6 +27,7 @@ class HomeRecommendationQueryService(
     private val snapshotPort: RecommendationSnapshotPort,
     private val aiCharacterSnapshotFallbackService: AiCharacterSnapshotFallbackPort? = null,
     private val cheerCreatorSnapshotFallbackService: CheerCreatorSnapshotFallbackPort? = null,
+    private val popularCommunitySnapshotFallbackService: PopularCommunitySnapshotFallbackPort? = null,
     private val clock: Clock = Clock.systemUTC()
 ) {
     private val snapshotWindowPolicy = RecommendationSnapshotWindowPolicy()
@@ -129,8 +130,7 @@ class HomeRecommendationQueryService(
         memberId: Long? = null,
         includeAdultCommunities: Boolean = false
     ): List<HomePopularCommunityRecommendationRecord> {
-        val snapshots = snapshotPort.findLatestSnapshots(RecommendedSectionType.POPULAR_COMMUNITY)
-            .take(POPULAR_COMMUNITY_CANDIDATE_LIMIT)
+        val snapshots = findPopularCommunitySnapshotsWithFallback()
         val detailsById = queryPort.findPopularCommunityRecommendationDetails(
             snapshots.map { it.targetId },
             memberId,
@@ -144,6 +144,21 @@ class HomeRecommendationQueryService(
         }.take(limit)
     }
 
+    private fun findPopularCommunitySnapshotsWithFallback(): List<RecommendationSnapshotRecord> {
+        val nowUtc = LocalDateTime.now(clock)
+        val snapshotAt = snapshotWindowPolicy.previousKstDayUtcWindow(nowUtc).snapshotAt
+        val snapshots = snapshotPort.findSnapshots(
+            RecommendedSectionType.POPULAR_COMMUNITY,
+            snapshotAt,
+            limit = POPULAR_COMMUNITY_CANDIDATE_LIMIT
+        )
+        if (snapshots.isNotEmpty()) return snapshots
+        if (snapshotPort.existsSnapshot(RecommendedSectionType.POPULAR_COMMUNITY, snapshotAt)) return emptyList()
+        return popularCommunitySnapshotFallbackService
+            ?.refreshPopularCommunityIfMissing(offset = 0, limit = POPULAR_COMMUNITY_CANDIDATE_LIMIT, nowUtc = nowUtc)
+            .orEmpty()
+    }
+
     fun findGenreCreatorRecommendations(
         memberId: Long?,
         includeAdultGenres: Boolean,