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/errors/application-error.ts

@@ -0,0 +1,60 @@
+export enum CoreEduErrorCode {
+  VALIDATION_ERROR = 'CORE_EDU_VALIDATION_ERROR',
+  NOT_FOUND = 'CORE_EDU_NOT_FOUND',
+  UNAUTHORIZED = 'CORE_EDU_UNAUTHORIZED',
+  FORBIDDEN = 'CORE_EDU_FORBIDDEN',
+  CONFLICT = 'CORE_EDU_CONFLICT',
+  INTERNAL_ERROR = 'CORE_EDU_INTERNAL_ERROR',
+  EXAM_NOT_FOUND = 'CORE_EDU_EXAM_NOT_FOUND',
+  HOMEWORK_NOT_FOUND = 'CORE_EDU_HOMEWORK_NOT_FOUND',
+  GRADE_NOT_FOUND = 'CORE_EDU_GRADE_NOT_FOUND',
+  OUTBOX_PUBLISH_FAILED = 'CORE_EDU_OUTBOX_PUBLISH_FAILED',
+}
+
+export class ApplicationError extends Error {
+  constructor(
+    public readonly code: CoreEduErrorCode,
+    message: string,
+    public readonly statusCode: number = 500,
+    public readonly details?: unknown,
+  ) {
+    super(message);
+    this.name = 'ApplicationError';
+  }
+}
+
+export class ValidationError extends ApplicationError {
+  constructor(message: string, details?: unknown) {
+    super(CoreEduErrorCode.VALIDATION_ERROR, message, 400, details);
+  }
+}
+
+export class NotFoundError extends ApplicationError {
+  constructor(message: string, details?: unknown) {
+    super(CoreEduErrorCode.NOT_FOUND, message, 404, details);
+  }
+}
+
+export class UnauthorizedError extends ApplicationError {
+  constructor(message: string = 'Unauthorized') {
+    super(CoreEduErrorCode.UNAUTHORIZED, message, 401);
+  }
+}
+
+export class ForbiddenError extends ApplicationError {
+  constructor(message: string = 'Forbidden') {
+    super(CoreEduErrorCode.FORBIDDEN, message, 403);
+  }
+}
+
+export class ConflictError extends ApplicationError {
+  constructor(message: string, details?: unknown) {
+    super(CoreEduErrorCode.CONFLICT, message, 409, details);
+  }
+}
+
+export class InternalError extends ApplicationError {
+  constructor(message: string = 'Internal server error', details?: unknown) {
+    super(CoreEduErrorCode.INTERNAL_ERROR, message, 500, details);
+  }
+}

services/core-edu/src/shared/errors/global-error.filter.ts

@@ -0,0 +1,63 @@
+import {
+  ExceptionFilter,
+  Catch,
+  ArgumentsHost,
+  HttpException,
+  HttpStatus,
+} from '@nestjs/common';
+import { ZodError } from 'zod';
+import { ApplicationError, CoreEduErrorCode } from './application-error.js';
+import { logger } from '../observability/logger.js';
+
+@Catch()
+export class GlobalErrorFilter implements ExceptionFilter {
+  catch(exception: unknown, host: ArgumentsHost): void {
+    const ctx = host.switchToHttp();
+    const response = ctx.getResponse();
+    const request = ctx.getRequest();
+
+    let statusCode = HttpStatus.INTERNAL_SERVER_ERROR;
+    let code = CoreEduErrorCode.INTERNAL_ERROR;
+    let message = 'Internal server error';
+    let details: unknown;
+
+    if (exception instanceof ApplicationError) {
+      statusCode = exception.statusCode;
+      code = exception.code;
+      message = exception.message;
+      details = exception.details;
+    } else if (exception instanceof ZodError) {
+      statusCode = HttpStatus.BAD_REQUEST;
+      code = CoreEduErrorCode.VALIDATION_ERROR;
+      message = 'Validation failed';
+      details = exception.flatten().fieldErrors;
+    } else if (exception instanceof HttpException) {
+      statusCode = exception.getStatus();
+      const resp = exception.getResponse();
+      message =
+        typeof resp === 'string'
+          ? resp
+          : (resp as { message?: string }).message ?? exception.message;
+    } else if (exception instanceof Error) {
+      message = exception.message;
+    }
+
+    logger.error(
+      {
+        err: exception,
+        path: request.url,
+        method: request.method,
+        code,
+      },
+      `Request failed: ${message}`,
+    );
+
+    response.status(statusCode).json({
+      code,
+      message,
+      details,
+      timestamp: new Date().toISOString(),
+      path: request.url,
+    });
+  }
+}