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.repository.ts

@@ -0,0 +1,28 @@
+import { eq } from 'drizzle-orm';
+import { db } from '../../config/database.js';
+import { exams, type Exam, type NewExam } from './exams.schema.js';
+
+export class ExamsRepository {
+  async findById(id: string): Promise<Exam | undefined> {
+    const [result] = await db
+      .select()
+      .from(exams)
+      .where(eq(exams.id, id))
+      .limit(1);
+    return result;
+  }
+
+  async findByClassId(classId: string): Promise<Exam[]> {
+    return db.select().from(exams).where(eq(exams.classId, classId));
+  }
+
+  async update(id: string, data: Partial<NewExam>): Promise<void> {
+    await db.update(exams).set(data).where(eq(exams.id, id));
+  }
+
+  async delete(id: string): Promise<void> {
+    await db.delete(exams).where(eq(exams.id, id));
+  }
+}
+
+export const examsRepository = new ExamsRepository();