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/classes/src/app.module.ts

@@ -1,7 +1,8 @@
-import { Module } from '@nestjs/common';
-import { ClassesModule } from './classes/classes.module.js';
+import { Module } from "@nestjs/common";
+import { ClassesModule } from "./classes/classes.module.js";
+import { HealthModule } from "./shared/health/health.module.js";
 
 @Module({
-  imports: [ClassesModule],
+  imports: [ClassesModule, HealthModule],
 })
 export class AppModule {}