fix: 修复 Dockerfile 中迁移文件路径错误

- 更正 pkg/migration/migrations 路径为 pkg/migration
- 解决构建时找不到目录的问题
- 迁移文件直接在 pkg/migration 目录下，无需子目录

backend/Dockerfile

@@ -52,7 +52,7 @@ COPY --from=builder /app/migrate /migrate
 # 复制配置文件和脚本
 COPY --from=builder /app/configs /configs
 COPY --from=builder /app/scripts /scripts
-COPY --from=builder /app/pkg/migration/migrations /pkg/migration/migrations
+COPY --from=builder /app/pkg/migration /pkg/migration
 
 # 设置时区
 ENV TZ=Asia/Shanghai