fix(api,web): 下单/读取请求身份隔离与零手续费支持（3d-1 评审修正）

- Web: orderSeq 拆分为 orderCreateSeq/orderReadSeq，并发刷新不再丢弃在途下单结果；换基金/换 token 双序列失效并携带 fundId 双重守卫
- 零手续费：DB CHECK fee_amount >= 0（幂等迁移），校验改为拒绝负数，UI 提示与占位符同步
- 测试：API 增加零手续费回放与 DB 负手续费约束回归（43/43），浏览器新增 H 系列延迟 POST 并发刷新/失败恢复/换基金隔离（78/78）

1 files changed, 6 insertions, 3 deletions

diff --git a/src/FundLab.Api/Persistence.fs b/src/FundLab.Api/Persistence.fs
index 88c0350..6c60bdd 100644
--- a/src/FundLab.Api/Persistence.fs
+++ b/src/FundLab.Api/Persistence.fs
@@ -131,13 +131,16 @@ type FundRepository(connectionString: string) =
             fund_id uuid NOT NULL REFERENCES funds(id),
             fund_code text NOT NULL,
             amount numeric(20, 2) NOT NULL CHECK (amount > 0),
-            fee_amount numeric(20, 2) NOT NULL CHECK (fee_amount > 0),
+            fee_amount numeric(20, 2) NOT NULL CHECK (fee_amount >= 0),
             reserved_total numeric(20, 2) NOT NULL CHECK (reserved_total > 0),
             status text NOT NULL,
             is_synthetic boolean NOT NULL,
             submitted_at timestamptz NOT NULL DEFAULT now()
         );
 
+        ALTER TABLE subscription_orders DROP CONSTRAINT IF EXISTS subscription_orders_fee_amount_check;
+        ALTER TABLE subscription_orders ADD CONSTRAINT subscription_orders_fee_amount_check CHECK (fee_amount >= 0);
+
         CREATE TABLE IF NOT EXISTS subscription_order_idempotencies (
             idempotency_key text PRIMARY KEY,
             request_hash text NOT NULL,
@@ -564,8 +567,8 @@ type FundRepository(connectionString: string) =
             Error "fund code cannot be empty"
         elif command.Amount <= 0m then
             Error "amount must be positive"
-        elif command.FeeAmount <= 0m then
-            Error "fee amount must be positive"
+        elif command.FeeAmount < 0m then
+            Error "fee amount cannot be negative"
         elif Decimal.Round(command.Amount, 2) <> command.Amount then
             Error "amount exceeds cash precision"
         elif Decimal.Round(command.FeeAmount, 2) <> command.FeeAmount then