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/database.ts

@@ -0,0 +1,24 @@
+import { drizzle } from 'drizzle-orm/mysql2';
+import mysql from 'mysql2/promise';
+import { env } from './env.js';
+
+let pool: mysql.Pool | null = null;
+
+export function getDb() {
+  if (!pool) {
+    pool = mysql.createPool({
+      uri: env.DATABASE_URL,
+      waitForConnections: true,
+      connectionLimit: 10,
+      queueLimit: 0,
+    });
+  }
+  return drizzle(pool);
+}
+
+export async function closeDb(): Promise<void> {
+  if (pool) {
+    await pool.end();
+    pool = null;
+  }
+}

services/core-edu/src/config/env.ts

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+import { z } from 'zod';
+
+const envSchema = z.object({
+  PORT: z.string().default('3004'),
+  DATABASE_URL: z.string().url(),
+  REDIS_URL: z.string().url().optional(),
+  JWT_SECRET: z.string(),
+  JWT_ISSUER: z.string().default('next-edu-cloud'),
+  KAFKA_BROKERS: z.string().default('localhost:9092'),
+  OTEL_EXPORTER_OTLP_ENDPOINT: z.string().url().optional(),
+  LOG_LEVEL: z.enum(['fatal', 'error', 'warn', 'info', 'debug', 'trace']).default('info'),
+  NODE_ENV: z.enum(['development', 'production', 'test']).default('development'),
+});
+
+export type Env = z.infer<typeof envSchema>;
+
+export function loadEnv(): Env {
+  const result = envSchema.safeParse(process.env);
+  if (!result.success) {
+    console.error('❌ Invalid environment variables:', result.error.flatten().fieldErrors);
+    throw new Error('Invalid environment configuration');
+  }
+  return result.data;
+}
+
+export const env = loadEnv();

services/core-edu/src/config/kafka.ts

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+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();
+}