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/msg/tsconfig.json

@@ -8,6 +8,7 @@
     "emitDecoratorMetadata": true,
     "outDir": "./dist",
     "rootDir": "./src",
+    "incremental": false,
     "types": ["node"]
   },
   "include": ["src/**/*"],