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.service.ts

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+import { randomUUID } from 'node:crypto';
+import { eq } from 'drizzle-orm';
+import { Injectable } from '@nestjs/common';
+import { db } from '../../config/database.js';
+import { exams } from './exams.schema.js';
+import { examsRepository } from './exams.repository.js';
+import { outboxRepository } from '../../shared/outbox/outbox.repository.js';
+import type { Exam, NewExam } from './exams.schema.js';
+import { NotFoundError, ValidationError } from '../../shared/errors/application-error.js';
+
+export interface CreateExamInput {
+  classId: string;
+  title: string;
+  description?: string;
+  examDate: Date;
+  duration: string;
+  totalScore: string;
+  createdBy: string;
+}
+
+export interface UpdateExamInput {
+  title?: string;
+  description?: string;
+  examDate?: Date;
+  duration?: string;
+  totalScore?: string;
+  status?: string;
+}
+
+@Injectable()
+export class ExamsService {
+  async createExam(input: CreateExamInput): Promise<{ id: string }> {
+    if (!input.classId || !input.title || !input.createdBy) {
+      throw new ValidationError('classId, title, createdBy are required');
+    }
+
+    const id = randomUUID();
+    const exam: NewExam = {
+      id,
+      classId: input.classId,
+      title: input.title,
+      description: input.description,
+      examDate: input.examDate,
+      duration: input.duration,
+      totalScore: input.totalScore,
+      status: 'draft',
+      createdBy: input.createdBy,
+    };
+
+    await db.transaction(async (tx) => {
+      await tx.insert(exams).values(exam);
+      await outboxRepository.create(
+        {
+          id: randomUUID(),
+          aggregateId: id,
+          aggregateType: 'exam',
+          eventType: 'exam.created',
+          payload: JSON.stringify({
+            id,
+            classId: input.classId,
+            title: input.title,
+          }),
+          status: 'pending',
+        },
+        tx,
+      );
+    });
+
+    return { id };
+  }
+
+  async getExam(id: string): Promise<Exam> {
+    const exam = await examsRepository.findById(id);
+    if (!exam) {
+      throw new NotFoundError(`Exam ${id} not found`);
+    }
+    return exam;
+  }
+
+  async listExamsByClass(classId: string): Promise<Exam[]> {
+    return examsRepository.findByClassId(classId);
+  }
+
+  async updateExam(id: string, data: UpdateExamInput): Promise<void> {
+    const existing = await examsRepository.findById(id);
+    if (!existing) {
+      throw new NotFoundError(`Exam ${id} not found`);
+    }
+
+    await db.transaction(async (tx) => {
+      await tx.update(exams).set(data).where(eq(exams.id, id));
+      await outboxRepository.create(
+        {
+          id: randomUUID(),
+          aggregateId: id,
+          aggregateType: 'exam',
+          eventType: 'exam.updated',
+          payload: JSON.stringify({ id, changes: data }),
+          status: 'pending',
+        },
+        tx,
+      );
+    });
+  }
+
+  async deleteExam(id: string): Promise<void> {
+    const existing = await examsRepository.findById(id);
+    if (!existing) {
+      throw new NotFoundError(`Exam ${id} not found`);
+    }
+
+    await db.transaction(async (tx) => {
+      await tx.delete(exams).where(eq(exams.id, id));
+      await outboxRepository.create(
+        {
+          id: randomUUID(),
+          aggregateId: id,
+          aggregateType: 'exam',
+          eventType: 'exam.deleted',
+          payload: JSON.stringify({ id }),
+          status: 'pending',
+        },
+        tx,
+      );
+    });
+  }
+}