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>

monitoring/grafana/provisioning/datasources/datasource.yml

@@ -0,0 +1,9 @@
+apiVersion: 1
+
+datasources:
+  - name: Prometheus
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    isDefault: true
+    editable: true