feat: 技能库改为数据库存储 (Prisma Skill 模型 + seed)

- Prisma 新增 Skill 模型 (id/name/description/icon/category/difficulty/systemPrompt/starters/tasks/tags/prerequisiteIds)
- SkillsService 从 DB 读取替代硬编码常量
- seed.ts 新增 seedSkills() 写入 8 个技能
- SkillsController 保持不变，API 返回 JSON 解析后的 starters/tasks

backend/prisma/schema.prisma

@@ -263,6 +263,26 @@ model AiModel {
   @@map("ai_models")
 }
 
+model Skill {
+  id             String   @id
+  name           String
+  description    String
+  icon           String
+  category       String
+  difficulty     String
+  systemPrompt   String   @db.Text
+  starters       String   @db.Text
+  tasks          String   @db.Text
+  tags           String
+  prerequisiteIds String?
+  sortOrder      Int      @default(0)
+  status         String   @default("ACTIVE")
+  createdAt      DateTime @default(now())
+  updatedAt      DateTime @updatedAt
+
+  @@map("skills")
+}
+
 model SandboxSession {
   id             Int      @id @default(autoincrement())
   userId         Int