fix(aihr): make daily score migration backward compatible

backend/script/sql/update/aihr_20260714_daily_drill_score_mode_mysql8.sql

@@ -6,6 +6,22 @@ SET @table_exists = (
   WHERE table_schema = @db_name
     AND table_name = 'aihr_practice_assignment'
 );
+SET @score_column_exists = (
+  SELECT COUNT(*)
+  FROM information_schema.columns
+  WHERE table_schema = @db_name
+    AND table_name = 'aihr_practice_assignment'
+    AND column_name = 'score'
+);
+SET @score_ddl = IF(
+  @table_exists = 1 AND @score_column_exists = 0,
+  'ALTER TABLE aihr_practice_assignment ADD COLUMN score int DEFAULT NULL COMMENT ''每日三题得分''',
+  'SELECT 1'
+);
+PREPARE score_stmt FROM @score_ddl;
+EXECUTE score_stmt;
+DEALLOCATE PREPARE score_stmt;
+
 SET @column_exists = (
   SELECT COUNT(*)
   FROM information_schema.columns