refactor(课堂任务): 「课堂看板」改名「课堂任务」，地址换成 /task；班级 PK 换成 /pk

- 界面上的「课堂看板」「课堂作业」统一叫「课堂任务」，顶栏、老师首页、后台菜单一起改
- 前端 oj/classroom → oj/task、oj/class → oj/pk，两个名字不再只差几个字母却毫不相干；旧地址不留跳转
- 后端 routes/classroom.ts 拆开：课堂任务的接口进 routes/task.ts（/task/board|lesson|today），
  首页的「上次来」「班里在做」进 routes/home.ts；共用的取题口径挪进 services/lesson.ts
- 契约 classroom.ts 同样拆成 task.ts / home.ts，ClassBoard* 改叫 TaskBoard*

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/src/index.ts

@@ -10,7 +10,8 @@ import { authRoutes } from './routes/auth'
 import { accountRoutes } from './routes/account'
 import { contestRoutes } from './routes/contest'
 import { contentRoutes } from './routes/content'
-import { classroomRoutes } from './routes/classroom'
+import { homeRoutes } from './routes/home'
+import { taskRoutes } from './routes/task'
 import { rankingRoutes } from './routes/ranking'
 import { problemsetRoutes } from './routes/problemset'
 import { achievementRoutes } from './routes/achievement'
@@ -32,7 +33,8 @@ app.route('/api', accountRoutes)
 app.route('/api', siteRoutes)
 app.route('/api', contestRoutes)
 app.route('/api', contentRoutes)
-app.route('/api', classroomRoutes)
+app.route('/api', homeRoutes)
+app.route('/api', taskRoutes)
 app.route('/api', rankingRoutes)
 app.route('/api', problemsetRoutes)
 app.route('/api', achievementRoutes)