feat(observability): add Grafana with provisioned datasources and dashboards

Add obs-grafana service (grafana/grafana-oss:11.6.1) to docker-compose.observability.yml.
Datasources (Prometheus, Loki, Tempo) are auto-provisioned via
infra/observability/grafana/provisioning/datasources/datasources.yml with
cross-datasource linking (Loki traceId → Tempo, Tempo → Loki, service map via Prometheus).
Three dashboards are pre-loaded: Node Exporter Full (1860), Spring Boot Observability (17175),
Loki Logs (13639) — datasource template variables replaced with provisioned UIDs.
GRAFANA_ADMIN_PASSWORD added to .env.example.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/observability/grafana/provisioning/dashboards/dashboards.yml

@@ -0,0 +1,10 @@
+apiVersion: 1
+
+providers:
+  - name: default
+    type: file
+    disableDeletion: true
+    updateIntervalSeconds: 30
+    options:
+      path: /etc/grafana/provisioning/dashboards
+      foldersFromFilesStructure: false