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/shared/outbox/outbox.schema.ts

@@ -0,0 +1,16 @@
+import { mysqlTable, varchar, text, timestamp, char, bigint } from 'drizzle-orm/mysql-core';
+
+export const outbox = mysqlTable('core_edu_outbox', {
+  id: char('id', { length: 36 }).notNull().primaryKey(),
+  aggregateId: char('aggregate_id', { length: 36 }).notNull(),
+  aggregateType: varchar('aggregate_type', { length: 50 }).notNull(),
+  eventType: varchar('event_type', { length: 100 }).notNull(),
+  payload: text('payload').notNull(),
+  status: varchar('status', { length: 20 }).notNull().default('pending'),
+  retryCount: bigint('retry_count', { mode: 'number' }).notNull().default(0),
+  createdAt: timestamp('created_at').notNull().defaultNow(),
+  processedAt: timestamp('processed_at'),
+});
+
+export type OutboxMessage = typeof outbox.$inferSelect;
+export type NewOutboxMessage = typeof outbox.$inferInsert;