docs: prd 폴더와 sample-prd 추가, 기존 계획 문서 plan-task 폴더 아래로 이동

docs/plan-task/20260402_audio_content_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 = 'content_banner'
+      AND column_name = 'lang'
+);
+
+SET @add_lang_column_sql := IF(
+    @lang_column_exists = 0,
+    'ALTER TABLE content_banner ADD COLUMN lang VARCHAR(10) NULL COMMENT ''배너 노출 언어'' AFTER type',
+    'SELECT ''content_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 content_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 = 'content_banner'
+      AND column_name = 'lang'
+);
+
+SET @alter_lang_column_sql := IF(
+    @lang_column_nullable = 'YES',
+    'ALTER TABLE content_banner MODIFY COLUMN lang VARCHAR(10) NOT NULL DEFAULT ''KO'' COMMENT ''배너 노출 언어 (KO 기본, EN/JA 추가 가능)''',
+    'SELECT ''content_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;