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/observability/logger.ts

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+import pino from 'pino';
+import { env } from '../../config/env.js';
+
+export const logger = pino({
+  name: 'core-edu',
+  level: env.LOG_LEVEL,
+  base: { service: 'core-edu' },
+  ...(env.NODE_ENV === 'development'
+    ? {
+        transport: {
+          target: 'pino-pretty',
+          options: { colorize: true, translateTime: 'SYS:standard' },
+        },
+      }
+    : {}),
+});

services/core-edu/src/shared/observability/metrics.ts

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+import {
+  Registry,
+  Counter,
+  Histogram,
+  Gauge,
+  collectDefaultMetrics,
+} from 'prom-client';
+
+export const registry = new Registry();
+collectDefaultMetrics({ register: registry });
+
+export const httpRequestCounter = new Counter({
+  name: 'core_edu_requests_total',
+  help: 'Total HTTP requests',
+  labelNames: ['method', 'route', 'status'] as const,
+});
+registry.registerMetric(httpRequestCounter);
+
+export const httpRequestDuration = new Histogram({
+  name: 'core_edu_request_duration_seconds',
+  help: 'HTTP request duration in seconds',
+  labelNames: ['method', 'route', 'status'] as const,
+  buckets: [0.005, 0.01, 0.05, 0.1, 0.5, 1, 5],
+});
+registry.registerMetric(httpRequestDuration);
+
+export const outboxPendingGauge = new Gauge({
+  name: 'core_edu_outbox_pending',
+  help: 'Number of pending outbox messages',
+});
+registry.registerMetric(outboxPendingGauge);
+
+export const outboxPublishedCounter = new Counter({
+  name: 'core_edu_outbox_published_total',
+  help: 'Total outbox messages published to Kafka',
+  labelNames: ['eventType', 'topic'] as const,
+});
+registry.registerMetric(outboxPublishedCounter);

services/core-edu/src/shared/observability/tracer.ts

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+import { NodeSDK } from '@opentelemetry/sdk-node';
+import { OTLPTraceExporter } from '@opentelemetry/exporter-trace-otlp-http';
+import { env } from '../../config/env.js';
+import { logger } from './logger.js';
+
+let sdk: NodeSDK | undefined;
+
+export function initTracer(): void {
+  if (!env.OTEL_EXPORTER_OTLP_ENDPOINT) {
+    logger.warn('OTEL_EXPORTER_OTLP_ENDPOINT not set, tracing disabled');
+    return;
+  }
+  sdk = new NodeSDK({
+    serviceName: 'core-edu',
+    traceExporter: new OTLPTraceExporter({
+      url: `${env.OTEL_EXPORTER_OTLP_ENDPOINT}/v1/traces`,
+    }),
+  });
+  sdk.start();
+  logger.info('OpenTelemetry tracer initialized');
+}
+
+export async function shutdownTracer(): Promise<void> {
+  if (sdk) {
+    await sdk.shutdown();
+    sdk = undefined;
+    logger.info('OpenTelemetry tracer shutdown');
+  }
+}