feat(chat-banner): 다국어 캐릭터 배너 등록과 노출을 지원한다

배너를 언어별로 저장하고 요청 언어 우선 조회 후 한국어로 fallback 하도록 맞춘다.

docs/20260402_chat_character_banner_lang_ddl.sql

@@ -0,0 +1,41 @@
+SET @schema_name := DATABASE();
+
+SET @lang_column_exists := (
+    SELECT COUNT(1)
+    FROM information_schema.columns
+    WHERE table_schema = @schema_name
+      AND table_name = 'chat_character_banner'
+      AND column_name = 'lang'
+);
+
+SET @add_lang_column_sql := IF(
+    @lang_column_exists = 0,
+    'ALTER TABLE chat_character_banner ADD COLUMN lang VARCHAR(10) NULL COMMENT ''배너 노출 언어'' AFTER sort_order',
+    'SELECT ''chat_character_banner.lang already exists'' AS message'
+);
+
+PREPARE add_lang_column_stmt FROM @add_lang_column_sql;
+EXECUTE add_lang_column_stmt;
+DEALLOCATE PREPARE add_lang_column_stmt;
+
+UPDATE chat_character_banner
+SET lang = 'KO'
+WHERE lang IS NULL;
+
+SET @lang_column_nullable := (
+    SELECT IS_NULLABLE
+    FROM information_schema.columns
+    WHERE table_schema = @schema_name
+      AND table_name = 'chat_character_banner'
+      AND column_name = 'lang'
+);
+
+SET @alter_lang_column_sql := IF(
+    @lang_column_nullable = 'YES',
+    'ALTER TABLE chat_character_banner MODIFY COLUMN lang VARCHAR(10) NOT NULL DEFAULT ''KO'' COMMENT ''배너 노출 언어 (KO 기본, EN/JA 추가 가능)''',
+    'SELECT ''chat_character_banner.lang already normalized'' AS message'
+);
+
+PREPARE alter_lang_column_stmt FROM @alter_lang_column_sql;
+EXECUTE alter_lang_column_stmt;
+DEALLOCATE PREPARE alter_lang_column_stmt;