fix(seeds): 修复 clear_shop.sql collation 冲突

用 BINARY 比较绕过 utf8mb4_unicode_ci 与 utf8mb4_0900_ai_ci 的冲突。
shop code 均为纯 ASCII，BINARY 比较结果完全一致。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/seeds/clear_shop.sql

@@ -4,7 +4,7 @@
 -- 或由脚本拼接:
 --   echo "SET @shop_code='S001';" | cat - clear_shop.sql | mysql ...
 
-SET @sid = (SELECT id FROM shops WHERE code = @shop_code LIMIT 1);
+SET @sid = (SELECT id FROM shops WHERE BINARY code = BINARY @shop_code LIMIT 1);
 
 SET FOREIGN_KEY_CHECKS = 0;