fix(fulfillment): 履约回填改用 keyset 分页并补充待回填部分索引

- 迁移 108：order_items (id) WHERE fulfillment_mode IS NULL 部分索引
  生产已 CONCURRENTLY 预建；取 5000 条待回填 id 4.8s → 4.9ms
- reconcilePendingFulfillmentModes 增加 afterId / lastScannedId，批内固定 id > 游标
- 新增 resolveReconcileCursor：游标单调前进，未前进或不足一批即结束
- backfill-fulfillment-mode 支持 --afterId 断点续跑

apps/backend/scripts/backfill-fulfillment-mode.ts

@@ -9,6 +9,7 @@ import {
 const options = parseOptions(process.argv.slice(2))
 const batchSize = normalizeInteger(options.batchSize, 5000, 1, 50_000)
 const maxBatches = normalizeInteger(options.maxBatches, 1000, 1, 10_000)
+const afterId = normalizeInteger(options.afterId, 0, 0, Number.MAX_SAFE_INTEGER)
 
 async function main() {
   const column = await query<{ exists: boolean }>(
@@ -32,6 +33,7 @@ async function main() {
     batchSize,
     maxBatches,
     reason: 'backfill_2026_10_23',
+    afterId,
   })
   const counts = await getFulfillmentModeCounts()