feat(monitoring): add Prometheus metrics endpoint and Grafana dashboards

Add observability stack with @willsoto/nestjs-prometheus for /metrics endpoint,
Prometheus scraping config, and 4 auto-provisioned Grafana dashboards
(API overview, database, search, business metrics).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/package.json

@@ -23,6 +23,7 @@
     "@nestjs/throttler": "^6.5.0",
     "@paralleldrive/cuid2": "^3.3.0",
     "@prisma/client": "^6.0.0",
+    "@willsoto/nestjs-prometheus": "^6.1.0",
     "bcrypt": "^6.0.0",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.15.1",
@@ -36,6 +37,7 @@
     "passport-local": "^1.0.0",
     "pino": "^9.0.0",
     "pino-pretty": "^13.0.0",
+    "prom-client": "^15.1.3",
     "reflect-metadata": "^0.2.0",
     "rxjs": "^7.8.0",
     "sanitize-html": "^2.17.2",