feat(infra): Tempo + Prometheus + Grafana observability backplane in compose (refs #122)

Three built images on the cg network with config baked in: Tempo (OTLP ingest,
ports 4317/4318), Prometheus, and Grafana with both datasources auto-provisioned.
Config lives in infra/observability/. The verify-observability step runs early in
CI verify-stack; the three containers are added to the failure log-dump list.
Tempo's Grafana plugin has no datasource /health method, so the check proves
Tempo reachability through Grafana's datasource proxy instead.

refs #122

infra/observability/prometheus/Dockerfile

@@ -0,0 +1,2 @@
+FROM prom/prometheus:v2.55.1
+COPY prometheus.yml /etc/prometheus/prometheus.yml

infra/observability/prometheus/prometheus.yml

@@ -0,0 +1,10 @@
+# Prometheus scrape config (S-16a, ADR-0023). For the backplane slice it scrapes
+# only itself; the .NET services' /metrics scrape targets are added in S-16c
+# (#124) when the services expose metrics.
+global:
+  scrape_interval: 15s
+
+scrape_configs:
+  - job_name: prometheus
+    static_configs:
+      - targets: ['localhost:9090']