feat(cost-center): 卡券商品创建即生成渠道规则，补齐成本项预设

- 新建 tpv: 商品时自动写 third_party_voucher 规则，避免新卡券 SKU 停在待分类
- 成本项预设新增「第三方卡券采购成本」

apps/backend/src/services/fulfillment/channel-assign-service.ts

@@ -553,6 +553,25 @@ export const CHANNEL_RULE_KEY_TYPES = MATCH_KEY_TYPES
 export const CHANNEL_RULE_SOURCES = RULE_SOURCES
 export const CHANNEL_VALUES = FULFILLMENT_CHANNELS
 
+/** 为单个卡券 SKU 建规则（商品创建时调用，幂等）。 */
+export async function ensureVoucherRuleForSku(skuCode: string): Promise<boolean> {
+  const key = String(skuCode || '').trim()
+  if (!key) return false
+  const result = await query<{ id: number }>(
+    `
+      INSERT INTO sku_channel_rules (
+        provider, shop_id, match_key_type, match_key, channel, priority,
+        source, enabled, note, created_at, updated_at
+      )
+      VALUES ('', '', 'sku_code', $1, 'third_party_voucher', 70, 'voucher', TRUE, '卡券商品自动生成', NOW(), NOW())
+      ON CONFLICT (provider, shop_id, match_key_type, match_key, channel) DO NOTHING
+      RETURNING id
+    `,
+    [key],
+  )
+  return (result.rowCount || 0) > 0
+}
+
 /**
  * 为卡券商品（tpv: 前缀）批量补渠道规则。
  *