feat(i18n): 번역 작업 큐와 언어 감지 캐시를 도입한다

조회 중 외부 번역 호출을 줄이고 누락 번역을 비동기 job으로 처리한다.

src/test/kotlin/kr/co/vividnext/sodalive/content/LanguageDetectionCacheServiceTest.kt

@@ -0,0 +1,42 @@
+package kr.co.vividnext.sodalive.content
+
+import kr.co.vividnext.sodalive.i18n.translation.SourceTextNormalizer
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+import org.mockito.Mockito
+
+class LanguageDetectionCacheServiceTest {
+    @Test
+    fun shouldReuseCachedLanguageDetectionForSameNormalizedText() {
+        val repository = Mockito.mock(LanguageDetectionResultRepository::class.java)
+        val service = LanguageDetectionCacheService(repository)
+        val sourceHash = SourceTextNormalizer.hash("Hello world")
+
+        Mockito.`when`(
+            repository.findBySourceHashAndProviderAndNormalizationVersion(
+                sourceHash = sourceHash,
+                provider = "papago",
+                normalizationVersion = SourceTextNormalizer.NORMALIZATION_VERSION
+            )
+        ).thenReturn(
+            LanguageDetectionResult(
+                sourceHash = sourceHash,
+                sourceTextSample = "Hello world",
+                detectedLanguage = "en",
+                provider = "papago",
+                confidence = null,
+                normalizationVersion = SourceTextNormalizer.NORMALIZATION_VERSION
+            )
+        )
+
+        var providerCalls = 0
+        val detected = service.detectWithCache("Hello   world") {
+            providerCalls++
+            "ko"
+        }
+
+        assertEquals("en", detected)
+        assertEquals(0, providerCalls)
+        Mockito.verify(repository, Mockito.never()).save(Mockito.any(LanguageDetectionResult::class.java))
+    }
+}