feat(p3): core teaching service with Outbox + Kafka event bus

P3 阶段交付物：
- services/core-edu: 教学核心服务（DDD 限界上下文：exams/grades/homework/classes）
  - exams: 考试 CRUD + 事务内写 exam + outbox
  - grades: 成绩 CRUD
  - homework: 作业 CRUD
  - classes.module: 复用 P1 classes 模块（聚合到 core-edu 服务）
- Outbox 模式实现：
  - outbox.schema.ts: core_edu_outbox 表（id/aggregate_id/event_type/payload/status/retry_count）
  - outbox.repository.ts: 支持事务参数 tx，确保业务+事件原子性
  - outbox.publisher.ts: Kafka idempotent producer + transactionalId，TOPIC_MAP 路由 9 种事件，MAX_RETRY=5
- config/kafka.ts: idempotent producer + transactionalId 配置
- main.ts: 启动顺序 initTracer → connectKafka → outboxPublisher.start → app.listen
- packages/shared-proto/proto/core_edu.proto: ExamService/HomeworkService/GradeService 契约
- packages/shared-proto/proto/events.proto: ClassEvent/ExamEvent/HomeworkEvent/GradeEvent 领域事件契约

services/core-edu/src/exams/exams.controller.ts

@@ -0,0 +1,57 @@
+import {
+  Body,
+  Controller,
+  Delete,
+  Get,
+  Param,
+  Post,
+  Put,
+  UseGuards,
+} from '@nestjs/common';
+import { ExamsService, type CreateExamInput, type UpdateExamInput } from './exams.service.js';
+import { PermissionGuard, Permissions } from '../../middleware/permission.guard.js';
+
+interface SuccessResponse<T> {
+  data: T;
+  timestamp: string;
+}
+
+@Controller('api/v1/exams')
+export class ExamsController {
+  constructor(private readonly examsService: ExamsService) {}
+
+  @Post()
+  @UseGuards(new PermissionGuard(Permissions.EXAM_CREATE))
+  async create(@Body() body: CreateExamInput): Promise<SuccessResponse<{ id: string }>> {
+    const result = await this.examsService.createExam(body);
+    return { data: result, timestamp: new Date().toISOString() };
+  }
+
+  @Get(':id')
+  @UseGuards(new PermissionGuard(Permissions.EXAM_READ))
+  async findOne(@Param('id') id: string): Promise<SuccessResponse<Awaited<ReturnType<ExamsService['getExam']>>>> {
+    const data = await this.examsService.getExam(id);
+    return { data, timestamp: new Date().toISOString() };
+  }
+
+  @Get('class/:classId')
+  @UseGuards(new PermissionGuard(Permissions.EXAM_READ))
+  async listByClass(@Param('classId') classId: string): Promise<SuccessResponse<Awaited<ReturnType<ExamsService['listExamsByClass']>>>> {
+    const data = await this.examsService.listExamsByClass(classId);
+    return { data, timestamp: new Date().toISOString() };
+  }
+
+  @Put(':id')
+  @UseGuards(new PermissionGuard(Permissions.EXAM_UPDATE))
+  async update(@Param('id') id: string, @Body() body: UpdateExamInput): Promise<SuccessResponse<{ success: true }>> {
+    await this.examsService.updateExam(id, body);
+    return { data: { success: true }, timestamp: new Date().toISOString() };
+  }
+
+  @Delete(':id')
+  @UseGuards(new PermissionGuard(Permissions.EXAM_DELETE))
+  async remove(@Param('id') id: string): Promise<SuccessResponse<{ success: true }>> {
+    await this.examsService.deleteExam(id);
+    return { data: { success: true }, timestamp: new Date().toISOString() };
+  }
+}