feat(dev-deploy): full observability stack (Prometheus/Grafana/Loki/Tempo)

Stand up a production-mirror monitoring stack in the long-lived dev
contour, all on galaxy-dev-internal with no host ports (reached only via
the in-repo galaxy-dev-caddy):

- Prometheus scrapes backend:9100, gateway:9191, node-exporter and
  cadvisor (30s interval, 15d retention); Loki (7d) + promtail (Docker
  service discovery by the galaxy.stack=dev-deploy label) for logs;
  Tempo (3d) for traces.
- Backend and gateway now export OTLP traces to Tempo over plaintext
  gRPC on the internal network (OTEL_EXPORTER_OTLP_INSECURE).
- Grafana provisioned as code (Prometheus/Loki/Tempo datasources plus a
  starter dashboard), served under /grafana/ via Caddy sub-path mode;
  admin password from the GALAXY_DEV_GRAFANA_ADMIN_PASSWORD secret.
- Expose the Mailpit capture UI under /mailpit/ (Caddy basic-auth +
  MP_WEBROOT) so every captured message is readable regardless of relay.
- dev-deploy.yaml seeds the monitoring config to a stable, reboot-
  surviving host path and injects the Grafana admin secret.

Per-service memory limits keep the footprint within budget. All
collector config lives under tools/dev-deploy/monitoring/ for dev/prod
parity.

tools/dev-deploy/monitoring/prometheus/prometheus.yml

@@ -0,0 +1,24 @@
+# Prometheus scrape config for the dev observability stack. Retention is
+# a CLI flag in the compose command, not here. Targets are reached by
+# Docker DNS (compose service names) on galaxy-dev-internal; nothing is
+# published to the host.
+global:
+  scrape_interval: 30s
+  evaluation_interval: 30s
+
+scrape_configs:
+  - job_name: backend
+    static_configs:
+      - targets: ["galaxy-backend:9100"]
+  - job_name: gateway
+    static_configs:
+      - targets: ["galaxy-api:9191"]
+  - job_name: node
+    static_configs:
+      - targets: ["galaxy-node-exporter:9100"]
+  - job_name: cadvisor
+    static_configs:
+      - targets: ["galaxy-cadvisor:8080"]
+  - job_name: prometheus
+    static_configs:
+      - targets: ["localhost:9090"]