fix(infra): resolve NestJS dist build and Prometheus target issues

NestJS: disable incremental in 6 services tsconfig.json to fix dist
not emitted when nest-cli deleteOutDir conflicts with tsc tsbuildinfo.
classes/iam: import HealthModule in AppModule to fix /healthz 404.
classes: rewrite HealthController to Drizzle getDb from TypeORM DI.
teacher-bff: add /metrics endpoint for Prometheus scraping.
infra: add node/mysql/redis exporters to observability profile.
mysql-exporter v0.15.1 uses command-line flags not DATA_SOURCE_NAME.
prometheus: enable web.enable-lifecycle for hot reload.

services/teacher-bff/src/main.ts

@@ -3,6 +3,7 @@ import { NestFactory } from "@nestjs/core";
 import { AppModule } from "./app.module.js";
 import { env } from "./config/env.js";
 import { initTracer, shutdownTracer } from "./shared/observability/tracer.js";
+import { metricsRegistry } from "./shared/observability/metrics.js";
 
 async function bootstrap(): Promise<void> {
   initTracer();
@@ -12,6 +13,12 @@ async function bootstrap(): Promise<void> {
 
   app.enableShutdownHooks();
 
+  // Prometheus 指标端点：不鉴权，供 Prometheus 抓取。
+  app.getHttpAdapter().get("/metrics", async (_req, res) => {
+    res.set("Content-Type", metricsRegistry.contentType);
+    res.end(await metricsRegistry.metrics());
+  });
+
   await app.listen(env.PORT);
   console.log(`Teacher BFF started on port ${env.PORT}`);