feat(teacher-bff): 完整实现 teacher-bff GraphQL 聚合层

包含 clients/graphql/middleware、health probes、shared-ts contracts 等

services/teacher-bff/src/shared/observability/tracer.ts

@@ -9,7 +9,7 @@ export function initTracer(): void {
   if (!env.OTEL_EXPORTER_OTLP_ENDPOINT) return;
 
   sdk = new NodeSDK({
-    serviceName: "teacher-bff",
+    serviceName: env.OTEL_SERVICE_NAME,
     traceExporter: new OTLPTraceExporter({
       url: `${env.OTEL_EXPORTER_OTLP_ENDPOINT}/v1/traces`,
     }),
@@ -17,11 +17,12 @@ export function initTracer(): void {
   });
 
   sdk.start();
-  console.log("Tracer initialized with auto-instrumentations");
+  console.log(`Tracer initialized (service: ${env.OTEL_SERVICE_NAME})`);
 }
 
 export async function shutdownTracer(): Promise<void> {
   if (sdk) {
     await sdk.shutdown();
+    sdk = null;
   }
 }