重构: 手续费改为百分比/固定二选一，清理旧 Skin/Order/ShipLog 演示链路

手续费:
- 新增 fee_type 字段(rate/fixed)，百分比与固定金额二选一，不再叠加
- calculateServiceFee 按 fee_type 分支计算
- 商户创建/更新校验 fee_type，订单落库快照 fee_type
- 前端表单改为下拉选择手续费类型，动态显示对应输入框
- 测试拆为 TestFeeRate + TestFeeFixed

清理旧链路:
- 删除旧 Skin/Order/ShipLog 模型及 OrderService/SkinService
- Dashboard 迁移到 FulfillmentService
- 上游 /api/open/v1 改为基于 FulfillmentOrder 实现，接口契约不变
- 推送留痕改用 AuditLog，不再建 ShipLog 表
- 删除前端 Skins/Orders/ShipLogs/Distributors 页面及路由、菜单、API、类型
- 新增迁移 003: 添加 fee_type 列并 DROP 旧表

backend/internal/database/migrations/002_merchant_fee_features.sql

@@ -0,0 +1,22 @@
+ALTER TABLE users ALTER COLUMN role SET DEFAULT 'merchant';
+UPDATE users SET role = 'merchant' WHERE role = 'distributor';
+DROP INDEX IF EXISTS idx_users_invite_code;
+DROP INDEX IF EXISTS idx_users_parent_id;
+ALTER TABLE users DROP COLUMN IF EXISTS invite_code;
+ALTER TABLE users DROP COLUMN IF EXISTS parent_id;
+
+ALTER TABLE merchants ADD COLUMN IF NOT EXISTS features TEXT NOT NULL DEFAULT 'products,orders,wallet,api,callbacks';
+ALTER TABLE merchants ADD COLUMN IF NOT EXISTS fee_rate_bp BIGINT NOT NULL DEFAULT 0;
+ALTER TABLE merchants ADD COLUMN IF NOT EXISTS fee_fixed_amount BIGINT NOT NULL DEFAULT 0;
+
+ALTER TABLE skins DROP COLUMN IF EXISTS commission;
+
+DROP INDEX IF EXISTS idx_orders_distributor_id;
+ALTER TABLE orders DROP COLUMN IF EXISTS distributor_id;
+ALTER TABLE orders DROP COLUMN IF EXISTS commission_amt;
+
+ALTER TABLE fulfillment_orders ADD COLUMN IF NOT EXISTS base_amount BIGINT NOT NULL DEFAULT 0;
+ALTER TABLE fulfillment_orders ADD COLUMN IF NOT EXISTS fee_rate_bp BIGINT NOT NULL DEFAULT 0;
+ALTER TABLE fulfillment_orders ADD COLUMN IF NOT EXISTS fee_fixed_amount BIGINT NOT NULL DEFAULT 0;
+ALTER TABLE fulfillment_orders ADD COLUMN IF NOT EXISTS service_fee_amount BIGINT NOT NULL DEFAULT 0;
+UPDATE fulfillment_orders SET base_amount = amount WHERE base_amount = 0;

backend/internal/database/migrations/003_drop_legacy_tables.sql

@@ -0,0 +1,9 @@
+-- 手续费改为"百分比或固定"二选一：新增 fee_type 列。
+ALTER TABLE merchants ADD COLUMN IF NOT EXISTS fee_type VARCHAR(16) NOT NULL DEFAULT 'rate';
+ALTER TABLE fulfillment_orders ADD COLUMN IF NOT EXISTS fee_type VARCHAR(16) NOT NULL DEFAULT 'rate';
+
+-- 清理旧演示链路遗留表：skins / orders / ship_logs。
+-- 这些表属于旧的"皮肤源头"演示模型，已被商户履约模型（merchant_products / fulfillment_orders）取代。
+DROP TABLE IF EXISTS ship_logs;
+DROP TABLE IF EXISTS orders;
+DROP TABLE IF EXISTS skins;