货币体系: 全平台统一使用积分(POINT)替代人民币(CNY)

后端:
- 模型 currency 默认值 CNY→POINT (MerchantProduct/WalletAccount/FulfillmentOrder)
- DashboardStats TotalSales/TotalFees 从 float64 改为 int64，去掉 /100.0 转换
- 上游 OpenOrderQuery.Amount 从 float64 改为 int64，直接返回积分整数
- 钱包/商品创建时 currency 默认 POINT

前端:
- 去掉 centsToYuan/yuanToCents 转换函数，金额直接用整数积分
- money() 显示从 ¥X.XX 改为 X 积分
- 商品售价/成本表单字段名改回 price_amount/cost_amount，precision 改 0
- 钱包调整表单 amount_yuan→amount，precision 改 0
- 手续费固定金额表单 precision 改 0，label 改积分
- 币种选项 CNY→POINT
- Dashboard 成交金额/手续费 suffix 元→积分，去掉 precision

数据库:
- 新增迁移 004: currency 默认值 CNY→POINT，存量数据更新

frontend/src/pages/MerchantCenter.tsx

@@ -212,11 +212,11 @@ export default function MerchantCenter() {
     setEditingProduct(null)
     productForm.resetFields()
     productForm.setFieldsValue({
-      currency: 'CNY',
+      currency: 'POINT',
       stock: -1,
       status: 'active',
-      price_yuan: 0,
-      cost_yuan: 0,
+      price_amount: 0,
+      cost_amount: 0,
     })
     setProductOpen(true)
   }
@@ -225,8 +225,6 @@ export default function MerchantCenter() {
     setEditingProduct(record)
     productForm.setFieldsValue({
       ...record,
-      price_yuan: centsToYuan(record.price_amount),
-      cost_yuan: centsToYuan(record.cost_amount),
     })
     setProductOpen(true)
   }
@@ -236,11 +234,7 @@ export default function MerchantCenter() {
     try {
       const payload = {
         ...values,
-        price_amount: yuanToCents(values.price_yuan),
-        cost_amount: yuanToCents(values.cost_yuan),
       }
-      delete payload.price_yuan
-      delete payload.cost_yuan
       if (editingProduct) {
         await merchantApi.updateProduct(editingProduct.id, payload)
         message.success('商品已更新')
@@ -259,7 +253,7 @@ export default function MerchantCenter() {
     const values = await walletForm.validateFields()
     try {
       const walletData = await merchantApi.adjustWallet({
-        amount: yuanToCents(values.amount_yuan),
+        amount: values.amount,
         idempotency_key: values.idempotency_key,
         note: values.note,
       })
@@ -497,7 +491,7 @@ export default function MerchantCenter() {
                   <Descriptions size="small" bordered column={3} style={{ flex: 1 }}>
                     <Descriptions.Item label="可用余额">{money(wallet?.available_balance ?? 0)}</Descriptions.Item>
                     <Descriptions.Item label="冻结余额">{money(wallet?.frozen_balance ?? 0)}</Descriptions.Item>
-                    <Descriptions.Item label="币种">{wallet?.currency || 'CNY'}</Descriptions.Item>
+                    <Descriptions.Item label="币种">{wallet?.currency || 'POINT'}</Descriptions.Item>
                   </Descriptions>
                   {canFinance && (
                     <Button
@@ -604,14 +598,14 @@ export default function MerchantCenter() {
             </Form.Item>
           </Space>
           <Space size="middle" style={{ width: '100%' }}>
-            <Form.Item name="price_yuan" label="售价" rules={[{ required: true }]} style={{ width: 180 }}>
-              <InputNumber min={0} precision={2} style={{ width: '100%' }} />
+            <Form.Item name="price_amount" label="售价（积分）" rules={[{ required: true }]} style={{ width: 180 }}>
+              <InputNumber min={0} precision={0} style={{ width: '100%' }} />
             </Form.Item>
-            <Form.Item name="cost_yuan" label="成本" style={{ width: 180 }}>
-              <InputNumber min={0} precision={2} style={{ width: '100%' }} />
+            <Form.Item name="cost_amount" label="成本（积分）" style={{ width: 180 }}>
+              <InputNumber min={0} precision={0} style={{ width: '100%' }} />
             </Form.Item>
             <Form.Item name="currency" label="币种" style={{ width: 180 }}>
-              <Select options={[{ value: 'CNY', label: 'CNY' }]} />
+              <Select options={[{ value: 'POINT', label: 'POINT（积分）' }]} />
             </Form.Item>
           </Space>
           <Space size="middle" style={{ width: '100%' }}>
@@ -635,8 +629,8 @@ export default function MerchantCenter() {
 
       <Modal title="钱包调整" open={walletOpen} onOk={submitWalletAdjust} onCancel={() => setWalletOpen(false)} destroyOnClose>
         <Form form={walletForm} layout="vertical" style={{ marginTop: 16 }}>
-          <Form.Item name="amount_yuan" label="调整金额" rules={[{ required: true }]}>
-            <InputNumber precision={2} style={{ width: '100%' }} placeholder="正数充值，负数扣减" />
+          <Form.Item name="amount" label="调整积分" rules={[{ required: true }]}>
+            <InputNumber precision={0} style={{ width: '100%' }} placeholder="正数充值，负数扣减" />
           </Form.Item>
           <Form.Item name="idempotency_key" label="幂等键" rules={[{ required: true }]}>
             <Input />
@@ -734,14 +728,6 @@ function SecretBlock({ appKey, secret }: { appKey?: string; secret: string }) {
   )
 }
 
-function yuanToCents(value?: number | null) {
-  return Math.round(Number(value || 0) * 100)
-}
-
-function centsToYuan(value?: number | null) {
-  return Number(((value || 0) / 100).toFixed(2))
-}
-
 function featuresToList(features?: string) {
   const list = (features || '')
     .split(/[,\s]+/)
@@ -768,7 +754,7 @@ function resolveEnabledTab(current: string, features?: string) {
 }
 
 function money(value?: number | null) {
-  return `¥${centsToYuan(value).toFixed(2)}`
+  return `${Number(value || 0)} 积分`
 }
 
 function moneyWithSign(value?: number | null) {