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/main.ts

@@ -0,0 +1,50 @@
+import { NestFactory } from '@nestjs/core';
+import { AppModule } from './app.module.js';
+import { env } from './config/env.js';
+import { connectKafka, disconnectKafka } from './config/kafka.js';
+import { outboxPublisher } from './shared/outbox/outbox.publisher.js';
+import { GlobalErrorFilter } from './shared/errors/global-error.filter.js';
+import { initTracer, shutdownTracer } from './shared/observability/tracer.js';
+import { logger } from './shared/observability/logger.js';
+
+async function bootstrap(): Promise<void> {
+  initTracer();
+
+  const app = await NestFactory.create(AppModule, { bufferLogs: true });
+  app.setGlobalPrefix('api');
+  app.useGlobalFilters(new GlobalErrorFilter());
+  app.enableShutdownHooks();
+
+  // Connect Kafka producer/consumer before starting the outbox publisher
+  await connectKafka();
+
+  // Start the transactional outbox publisher - polls pending messages
+  // and publishes them to Kafka topics defined in TOPIC_MAP.
+  await outboxPublisher.start();
+
+  await app.listen(env.PORT);
+  logger.info(
+    { port: env.PORT, service: 'core-edu' },
+    'CoreEdu service is listening',
+  );
+
+  process.on('SIGTERM', async () => {
+    logger.info('SIGTERM received, shutting down gracefully...');
+    await outboxPublisher.stop();
+    await disconnectKafka();
+    await shutdownTracer();
+    await app.close();
+    process.exit(0);
+  });
+
+  process.on('SIGINT', async () => {
+    logger.info('SIGINT received, shutting down gracefully...');
+    await outboxPublisher.stop();
+    await disconnectKafka();
+    await shutdownTracer();
+    await app.close();
+    process.exit(0);
+  });
+}
+
+void bootstrap();