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/grafana/provisioning/datasources/datasources.yml

@@ -0,0 +1,22 @@
+# Grafana datasources provisioned as code (dev↔prod parity). All reach
+# the collectors by Docker DNS (compose service names) on
+# galaxy-dev-internal.
+apiVersion: 1
+
+datasources:
+  - name: Prometheus
+    type: prometheus
+    access: proxy
+    uid: prometheus
+    url: http://galaxy-prometheus:9090
+    isDefault: true
+  - name: Loki
+    type: loki
+    access: proxy
+    uid: loki
+    url: http://galaxy-loki:3100
+  - name: Tempo
+    type: tempo
+    access: proxy
+    uid: tempo
+    url: http://galaxy-tempo:3200