feat: 兑换改为扣除指定枚数并新增倒计时确认弹窗

- 后端按 collectedAt 顺序扣除最早的 threshold 枚图章，保留剩余
- 前端新增二次确认弹窗，含工作人员提示与 5 秒倒计时防误触
- 后台日志"图章数"列改为"扣除枚数"

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/server/src/routes/redemption.ts

@@ -41,10 +41,19 @@ router.post("/redeem", requireAuth, async (req, res) => {
   }
 
   const redemption = await prisma.$transaction(async (tx) => {
-    const record = await tx.redemption.create({
-      data: { userId: req.userId!, ruleId: rule.id, stampCount: collectionCount },
+    // Deduct the oldest N collections (chronological order by collectedAt)
+    const toDelete = await tx.collection.findMany({
+      where: { userId: req.userId! },
+      orderBy: { collectedAt: "asc" },
+      take: rule.threshold,
+      select: { id: true },
+    });
+    await tx.collection.deleteMany({
+      where: { id: { in: toDelete.map((c) => c.id) } },
+    });
+    const record = await tx.redemption.create({
+      data: { userId: req.userId!, ruleId: rule.id, stampCount: rule.threshold },
     });
-    await tx.collection.deleteMany({ where: { userId: req.userId! } });
     return record;
   });