P8 平台轻量化改造 + SSG修复 + 编程导师 + 文档完善

- Prisma: Tool 模型加 affiliateLink；免费用户沙盒 10→5 次/日
- 后端: Tools API + /admin/tools CRUD 5 端点；Practices 完整模块
- 导航: 主菜单隐藏企业版/社区（URL 可访问）
- 首页: 重定位为 AI 工具指南；新增精选工具区块；Feature 重写
- 工具页: affiliateLink 绿色推荐 Badge
- SSG 修复: config.ts 构建时直连 localhost:4000，页面 108→127
- 沙盒: 新增编程导师场景（苏格拉底教学法）
- 练习系统: Practices 多场景练习（含结构化评分）
- 技能广场: 6 个付费 Skill（标题大师/回款助手等）
- 管理后台: Models/Posts/Practices CRUD 页面
- 文档: README + progress.md 全面更新；AGENTS.md 同步定位
- 清理: .env.example 移除；tsbuildinfo gitignore

backend/prisma/schema.prisma

@@ -44,7 +44,8 @@ model User {
   status       UserStatus @default(ACTIVE)
   memberPlan   MemberPlan @default(FREE)
   memberExpire DateTime?
-  sandboxDaily Int        @default(10)
+  sandboxDaily Int        @default(5)
+  sandboxExtra Int        @default(0)
   followerCount Int       @default(0)
   followingCount Int      @default(0)
   postCount     Int       @default(0)
@@ -68,6 +69,8 @@ model User {
   circleMemberships CircleMember[]
   organizationMemberships OrganizationMember[]
   notifications Notification[]
+  practiceSubmissions PracticeSubmission[]
+  userSkills UserSkill[]
 
   @@map("users")
 }
@@ -132,7 +135,7 @@ model Lesson {
   id        Int           @id @default(autoincrement())
   chapterId Int
   title     String
-  content   String?
+  content   String?       @db.Text
   videoUrl  String?
   duration  Int?
   sortOrder Int           @default(0)
@@ -207,6 +210,7 @@ model Tool {
   description String?
   url         String
   icon        String?
+  affiliateLink String?
   categoryId  Int?
   tags        String?
   isFeatured  Boolean       @default(false)
@@ -225,7 +229,7 @@ model Content {
   id            Int           @id @default(autoincrement())
   title         String
   summary       String?
-  content       String?
+  content       String?   @db.Text
   cover         String?
   categoryId    Int?
   tags          String?
@@ -276,11 +280,14 @@ model Skill {
   tasks          String   @db.Text
   tags           String
   prerequisiteIds String?
+  price          Float?   @default(0)
   sortOrder      Int      @default(0)
   status         String   @default("ACTIVE")
   createdAt      DateTime @default(now())
   updatedAt      DateTime @updatedAt
 
+  purchasedBy UserSkill[]
+
   @@map("skills")
 }
 
@@ -313,17 +320,34 @@ model Order {
   payChannel      String?
   transactionId   String?
   gatewayOrderId  String?
+  metadata        String?
   paidAt          DateTime?
   createdAt     DateTime    @default(now())
   updatedAt     DateTime    @updatedAt
 
-  user User @relation(fields: [userId], references: [id])
+  user       User         @relation(fields: [userId], references: [id])
+  userSkills UserSkill[]
 
   @@index([userId, status])
   @@index([status, createdAt])
   @@map("orders")
 }
 
+model UserSkill {
+  id           Int      @id @default(autoincrement())
+  userId       Int
+  skillId      String
+  orderId      Int?
+  purchasedAt  DateTime @default(now())
+
+  user  User   @relation(fields: [userId], references: [id], onDelete: Cascade)
+  skill Skill  @relation(fields: [skillId], references: [id])
+  order Order? @relation(fields: [orderId], references: [id])
+
+  @@unique([userId, skillId])
+  @@map("user_skills")
+}
+
 model Subscription {
   id        Int        @id @default(autoincrement())
   userId    Int
@@ -384,7 +408,7 @@ model Post {
   id           Int      @id @default(autoincrement())
   userId       Int
   title        String
-  content      String
+  content      String   @db.Text
   tags         String?
   circleId     Int?
   status       String   @default("PUBLISHED")
@@ -547,6 +571,52 @@ model Notification {
   @@map("notifications")
 }
 
+model PracticeQuestion {
+  id              Int      @id @default(autoincrement())
+  skillId         String?
+  scenario        String   @db.Text
+  title           String
+  description     String   @db.Text
+  instructions    String   @db.Text
+  difficulty      String   @default("BEGINNER")
+  category        String   @default("general")
+  expectedCriteria String  @db.Text
+  starterCode     String?  @db.Text
+  hint            String?  @db.Text
+  sortOrder       Int      @default(0)
+  status          String   @default("PUBLISHED")
+  attemptCount    Int      @default(0)
+  createdAt       DateTime @default(now())
+  updatedAt       DateTime @updatedAt
+
+  submissions PracticeSubmission[]
+
+  @@map("practice_questions")
+}
+
+model PracticeSubmission {
+  id             Int      @id @default(autoincrement())
+  userId         Int
+  questionId     Int
+  answer         String   @db.Text
+  score          Int?
+  maxScore       Int      @default(100)
+  feedback       String?  @db.Text
+  criteriaScores String?  @db.Text
+  duration       Int      @default(0)
+  status         String   @default("SUBMITTED")
+  submittedAt    DateTime @default(now())
+  scoredAt       DateTime?
+
+  question PracticeQuestion @relation(fields: [questionId], references: [id], onDelete: Cascade)
+  user     User             @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@index([userId])
+  @@index([questionId])
+  @@index([userId, status])
+  @@map("practice_submissions")
+}
+
 model Banner {
   id          Int       @id @default(autoincrement())
   title       String