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/config/kafka.ts

@@ -0,0 +1,25 @@
+import { Kafka } from 'kafkajs';
+import { env } from './env.js';
+
+export const kafka = new Kafka({
+  brokers: env.KAFKA_BROKERS.split(','),
+  clientId: 'core-edu-service',
+});
+
+export const producer = kafka.producer({
+  idempotent: true,
+  transactionalId: 'core-edu-tx',
+});
+
+export const consumer = kafka.consumer({ groupId: 'core-edu-group' });
+
+export async function connectKafka(): Promise<void> {
+  await producer.connect();
+  await consumer.connect();
+  console.log('Kafka connected');
+}
+
+export async function disconnectKafka(): Promise<void> {
+  await producer.disconnect();
+  await consumer.disconnect();
+}