feat(i18n): 번역 작업 큐와 언어 감지 캐시를 도입한다

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

src/main/kotlin/kr/co/vividnext/sodalive/content/LanguageDetectionCacheService.kt

@@ -0,0 +1,46 @@
+package kr.co.vividnext.sodalive.content
+
+import kr.co.vividnext.sodalive.i18n.translation.SourceTextNormalizer
+import org.springframework.stereotype.Service
+import org.springframework.transaction.annotation.Transactional
+
+@Service
+class LanguageDetectionCacheService(
+    private val languageDetectionResultRepository: LanguageDetectionResultRepository
+) {
+    @Transactional
+    fun detectWithCache(
+        query: String,
+        provider: String = DEFAULT_PROVIDER,
+        detector: () -> String?
+    ): String? {
+        val normalizedQuery = SourceTextNormalizer.normalize(query)
+        if (normalizedQuery.isBlank()) return null
+
+        val sourceHash = SourceTextNormalizer.hash(normalizedQuery)
+        val cached = languageDetectionResultRepository.findBySourceHashAndProviderAndNormalizationVersion(
+            sourceHash = sourceHash,
+            provider = provider,
+            normalizationVersion = SourceTextNormalizer.NORMALIZATION_VERSION
+        )
+        if (cached != null) return cached.detectedLanguage
+
+        val detectedLanguage = detector()?.takeIf { it.isNotBlank() } ?: return null
+        languageDetectionResultRepository.save(
+            LanguageDetectionResult(
+                sourceHash = sourceHash,
+                sourceTextSample = normalizedQuery.take(MAX_SAMPLE_LENGTH),
+                detectedLanguage = detectedLanguage.lowercase(),
+                provider = provider,
+                confidence = null,
+                normalizationVersion = SourceTextNormalizer.NORMALIZATION_VERSION
+            )
+        )
+        return detectedLanguage.lowercase()
+    }
+
+    companion object {
+        const val DEFAULT_PROVIDER = "papago"
+        private const val MAX_SAMPLE_LENGTH = 500
+    }
+}