feat(recommend): 크리에이터 데뷔 판정 정책을 추가한다

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/main/kotlin/kr/co/vividnext/sodalive/v2/recommend/domain/CreatorDebutPolicy.kt

@@ -0,0 +1,19 @@
+package kr.co.vividnext.sodalive.v2.recommend.domain
+
+import java.time.LocalDateTime
+import java.time.temporal.ChronoUnit
+
+class CreatorDebutPolicy {
+    fun resolveDebutAt(firstContentPublishedAt: LocalDateTime?, firstLiveAt: LocalDateTime?): LocalDateTime? {
+        return listOfNotNull(firstContentPublishedAt, firstLiveAt).minOrNull()
+    }
+
+    fun isNewCreator(debutAt: LocalDateTime?, now: LocalDateTime): Boolean {
+        if (debutAt == null) {
+            return false
+        }
+
+        val days = ChronoUnit.DAYS.between(debutAt.toLocalDate(), now.toLocalDate())
+        return days in 0..30
+    }
+}