refactor: 兑换机制改为一图章一奖品并引入库存

- 废弃 RedemptionRule（集 N 换 1），新增 Prize 表与 Stamp 1:1 关联
- Redemption 记录直接绑定到 stampId + prizeId + prizeName 快照
- 兑换事务用 updateMany + stock>0 条件作乐观锁
- 兑换后保留 Collection 记录，图章持续彩色点亮并标记"已兑换"
- 用户端入口改为点击已收集图章弹出兑换，库存为 0 时按钮禁用
- 管理后台删除 /admin/rules，奖品编辑嵌入 StampForm

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

prisma/schema.prisma

@@ -29,6 +29,21 @@ model Stamp {
   createdAt   DateTime     @default(now())
   updatedAt   DateTime     @updatedAt
   collections Collection[]
+  redemptions Redemption[]
+  prize       Prize?
+}
+
+model Prize {
+  id          String       @id @default(uuid())
+  stampId     String       @unique
+  name        String
+  description String?
+  stock       Int          @default(0)
+  enabled     Boolean      @default(true)
+  createdAt   DateTime     @default(now())
+  updatedAt   DateTime     @updatedAt
+  stamp       Stamp        @relation(fields: [stampId], references: [id], onDelete: Cascade)
+  redemptions Redemption[]
 }
 
 model Collection {
@@ -43,27 +58,18 @@ model Collection {
   @@index([userId])
 }
 
-model RedemptionRule {
-  id          String       @id @default(uuid())
-  name        String
-  description String?
-  threshold   Int
-  enabled     Boolean      @default(true)
-  sortOrder   Int          @default(0)
-  createdAt   DateTime     @default(now())
-  updatedAt   DateTime     @updatedAt
-  redemptions Redemption[]
-}
-
 model Redemption {
-  id         String         @id @default(uuid())
+  id         String   @id @default(uuid())
   userId     String
-  ruleId     String
-  stampCount Int
-  redeemedAt DateTime       @default(now())
-  user       User           @relation(fields: [userId], references: [id], onDelete: Cascade)
-  rule       RedemptionRule @relation(fields: [ruleId], references: [id])
+  stampId    String
+  prizeId    String
+  prizeName  String
+  redeemedAt DateTime @default(now())
+  user       User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  stamp      Stamp    @relation(fields: [stampId], references: [id])
+  prize      Prize    @relation(fields: [prizeId], references: [id])
 
+  @@unique([userId, stampId])
   @@index([userId])
 }