支持结账多轮协商与按实际消耗双比例结算

允许双方最多 6 轮修改结账方案；哈夫币按实际消耗与 buyer/seller 比例计价，打超从押金扣，押金不足禁止自动完结并引导人工争议。同步收紧结账明细展示密度。

backend/migrations/000033_checkout_multi_round.sql

@@ -0,0 +1,20 @@
+-- +goose Up
+ALTER TABLE order_checkouts
+  ADD COLUMN round_count INT NOT NULL DEFAULT 1 COMMENT '结账协商轮次（提交为第1轮）' AFTER status,
+  ADD COLUMN turn VARCHAR(16) NOT NULL DEFAULT 'owner' COMMENT '当前待处理方: owner/renter' AFTER round_count,
+  ADD COLUMN proposed_by BIGINT UNSIGNED NOT NULL DEFAULT 0 COMMENT '当前提案提出人' AFTER turn,
+  ADD COLUMN shortfall_cent BIGINT NOT NULL DEFAULT 0 COMMENT '押金不足以覆盖打超/赔付的差额(分)' AFTER owner_income_amount_cent,
+  ADD COLUMN overshoot_amount_cent BIGINT NOT NULL DEFAULT 0 COMMENT '超出预收租金的打超金额(分)' AFTER shortfall_cent;
+
+-- 历史单：已反价等待租客的，轮到租客
+UPDATE order_checkouts
+SET turn = 'renter', round_count = GREATEST(round_count, 2)
+WHERE status = 'countered';
+
+-- +goose Down
+ALTER TABLE order_checkouts
+  DROP COLUMN overshoot_amount_cent,
+  DROP COLUMN shortfall_cent,
+  DROP COLUMN proposed_by,
+  DROP COLUMN turn,
+  DROP COLUMN round_count;