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/datasources/datasources.yml

@@ -0,0 +1,38 @@
+apiVersion: 1
+
+datasources:
+  - name: Prometheus
+    type: prometheus
+    uid: prometheus
+    url: http://obs-prometheus:9090
+    isDefault: true
+    editable: false
+
+  - name: Loki
+    type: loki
+    uid: loki
+    url: http://obs-loki:3100
+    editable: false
+    jsonData:
+      derivedFields:
+        - name: TraceID
+          matcherRegex: '"traceId":"(\w+)"'
+          url: "${__value.raw}"
+          datasourceUid: tempo
+
+  - name: Tempo
+    type: tempo
+    uid: tempo
+    url: http://obs-tempo:3200
+    editable: false
+    jsonData:
+      tracesToLogsV2:
+        datasourceUid: loki
+        spanStartTimeShift: "-1m"
+        spanEndTimeShift: "1m"
+        filterByTraceID: true
+        filterBySpanID: false
+      serviceMap:
+        datasourceUid: prometheus
+      nodeGraph:
+        enabled: true