迁移去重与改号：消除与 main 的 000050-053 撞号

- 删除 dev 的 000051_user_token_version（main 的 000052_user_token_version 已实现同列）
- order_list_least_privilege / file_upload_owners / cleanup_index 改号为 000067-000069
- 000068 改为 CREATE TABLE IF NOT EXISTS、000069 改为条件加索引，兼容已执行旧编号的历史库

backend/migrations/000051_user_token_version.sql

@@ -1,10 +0,0 @@
--- +goose Up
-
--- 用户会话版本：冻结、改密和退出时递增，立即撤销旧 access/refresh token。
-ALTER TABLE users
-  ADD COLUMN token_version BIGINT NOT NULL DEFAULT 1 COMMENT '用户令牌版本';
-
--- +goose Down
-
-ALTER TABLE users
-  DROP COLUMN token_version;

backend/migrations/000053_file_upload_owners_cleanup_index.sql

@@ -1,10 +0,0 @@
--- +goose Up
-
--- 为长期未关联上传归属记录的定时清理提供索引。
-ALTER TABLE file_upload_owners
-  ADD KEY idx_file_upload_owners_created_at (created_at);
-
--- +goose Down
-
-ALTER TABLE file_upload_owners
-  DROP INDEX idx_file_upload_owners_created_at;

backend/migrations/000068_file_upload_owners.sql

@@ -1,7 +1,7 @@
 -- +goose Up
 
 -- 私有文件上传归属：在业务记录写入前，仅允许上传者预览自己的临时文件。
-CREATE TABLE file_upload_owners (
+CREATE TABLE IF NOT EXISTS file_upload_owners (
   id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
   user_id BIGINT UNSIGNED NOT NULL,
   object_key VARCHAR(512) NOT NULL,

backend/migrations/000069_file_upload_owners_cleanup_index.sql

@@ -0,0 +1,21 @@
+-- +goose Up
+
+-- 为长期未关联上传归属记录的定时清理提供索引。
+-- 兼容历史上已按旧编号执行过该索引的数据库，重复执行时跳过。
+SET @idx_exists := (
+  SELECT COUNT(*) FROM information_schema.statistics
+  WHERE table_schema = DATABASE()
+    AND table_name = 'file_upload_owners'
+    AND index_name = 'idx_file_upload_owners_created_at'
+);
+SET @ddl := IF(@idx_exists > 0,
+  'SELECT 1',
+  'ALTER TABLE file_upload_owners ADD KEY idx_file_upload_owners_created_at (created_at)');
+PREPARE stmt FROM @ddl;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+-- +goose Down
+
+ALTER TABLE file_upload_owners
+  DROP INDEX idx_file_upload_owners_created_at;