refactor(finance)!: 强制渠道模型，移除旧履约方式与同步链路

- 删除 fulfillment_mode 相关的旧同步/回填链路与定时任务
- 删除 cost_finance_sync、成本快照同步、人工商品归一等遗留服务
- 迁移 113 直接删除 order_items.fulfillment_mode 及其索引约束
- 断代脚本自带归档步骤，不再依赖独立回填脚本
- 退款/事实/总账统一走渠道版销售事实

apps/backend/package.json

@@ -11,8 +11,6 @@
     "db:migrate:status": "tsx scripts/migration-status.ts",
     "finance:sync": "tsx scripts/sync-cost-finance.ts",
     "finance:rebuild-cutover": "tsx scripts/rebuild-after-cutover.ts",
-    "fulfillment:backfill": "tsx scripts/backfill-fulfillment-mode.ts",
-    "fulfillment:backfill-channel": "tsx scripts/backfill-fulfillment-channel.ts",
     "revenue:backfill": "tsx scripts/backfill-revenue-basis.ts",
     "cost-exceptions:archive": "tsx scripts/archive-cost-exceptions.ts",
     "storage:migrate": "tsx scripts/migrate-storage.ts",