feat: 盲盒房间体系重构 — 强制登录、独立房间、用户归属

- 新增 BlindBoxRoom/BlindBoxMember 模型，BlindBoxIdea 增加 userId/drawnById
- 新增房间 API（创建/加入/列表/详情），所有盲盒 API 增加认证和成员校验
- 新建盲盒大厅页面（三层引导式设计：未登录氛围页/首次创建引导/房间列表）
- 新建盲盒房间页面（成员校验/邀请分享/用户归属展示/自动聚焦）
- 首页删除契约画廊和 localStorage 盲盒逻辑，周末契约跳转到 /blindbox
- 清理旧路由 /room/[id]/blindbox
- 提取共享工具 src/lib/blindbox.ts（错误响应/房间号生成/成员校验）
- AuthModal 支持 defaultTab 参数
- 更新项目规范：新项目原则、代码优雅和复用优先

prisma/schema.prisma

@@ -24,6 +24,11 @@ model User {
   createdAt    DateTime   @default(now())
   decisions    Decision[]
   favorites    Favorite[]
+
+  createdBlindBoxRooms BlindBoxRoom[]   @relation("RoomCreator")
+  blindBoxMemberships  BlindBoxMember[]
+  submittedIdeas       BlindBoxIdea[]   @relation("IdeaSubmitter")
+  drawnIdeas           BlindBoxIdea[]   @relation("IdeaDrawer")
 }
 
 model Decision {
@@ -46,10 +51,40 @@ model Favorite {
   user           User     @relation(fields: [userId], references: [id])
 }
 
+model BlindBoxRoom {
+  id        String   @id @default(cuid())
+  code      String   @unique
+  name      String
+  creatorId String
+  createdAt DateTime @default(now())
+
+  creator User             @relation("RoomCreator", fields: [creatorId], references: [id])
+  members BlindBoxMember[]
+  ideas   BlindBoxIdea[]
+}
+
+model BlindBoxMember {
+  id       String   @id @default(cuid())
+  roomId   String
+  userId   String
+  joinedAt DateTime @default(now())
+
+  room BlindBoxRoom @relation(fields: [roomId], references: [id], onDelete: Cascade)
+  user User         @relation(fields: [userId], references: [id])
+
+  @@unique([roomId, userId])
+}
+
 model BlindBoxIdea {
   id        String   @id @default(uuid())
   roomId    String
+  userId    String
   content   String
   status    String   @default("in_pool")
+  drawnById String?
   createdAt DateTime @default(now())
+
+  room    BlindBoxRoom @relation(fields: [roomId], references: [id], onDelete: Cascade)
+  user    User         @relation("IdeaSubmitter", fields: [userId], references: [id])
+  drawnBy User?        @relation("IdeaDrawer", fields: [drawnById], references: [id])
 }