fix(language-detection): 최초 감지 캐시 경합을 직렬화한다

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

@@ -25,18 +25,22 @@ class LanguageDetectionCacheService(
         )
         if (cached != null) return cached.detectedLanguage
 
-        val detectedLanguage = detector()?.takeIf { it.isNotBlank() } ?: return null
-        languageDetectionResultRepository.save(
-            LanguageDetectionResult(
+        val detectedLanguage = detector()?.takeIf { it.isNotBlank() }?.lowercase() ?: return null
+        languageDetectionResultRepository.insertIfAbsent(
+            sourceHash = sourceHash,
+            sourceTextSample = normalizedQuery.take(MAX_SAMPLE_LENGTH),
+            detectedLanguage = detectedLanguage,
+            provider = provider,
+            confidence = null,
+            normalizationVersion = SourceTextNormalizer.NORMALIZATION_VERSION
+        )
+        return requireNotNull(
+            languageDetectionResultRepository.findDetectedLanguageForUpdate(
                 sourceHash = sourceHash,
-                sourceTextSample = normalizedQuery.take(MAX_SAMPLE_LENGTH),
-                detectedLanguage = detectedLanguage.lowercase(),
                 provider = provider,
-                confidence = null,
                 normalizationVersion = SourceTextNormalizer.NORMALIZATION_VERSION
             )
         )
-        return detectedLanguage.lowercase()
     }
 
     companion object {