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/dashboards/galaxy-overview.json

@@ -0,0 +1,46 @@
+{
+  "annotations": { "list": [] },
+  "editable": true,
+  "graphTooltip": 0,
+  "panels": [
+    {
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "fieldConfig": { "defaults": { "unit": "reqps" }, "overrides": [] },
+      "gridPos": { "h": 8, "w": 12, "x": 0, "y": 0 },
+      "id": 1,
+      "title": "Backend HTTP request rate",
+      "type": "timeseries",
+      "targets": [
+        {
+          "datasource": { "type": "prometheus", "uid": "prometheus" },
+          "expr": "sum by (group) (rate(http_requests_total[5m]))",
+          "legendFormat": "{{group}}"
+        }
+      ]
+    },
+    {
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "fieldConfig": { "defaults": { "unit": "bytes" }, "overrides": [] },
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 0 },
+      "id": 2,
+      "title": "Container memory (cadvisor)",
+      "type": "timeseries",
+      "targets": [
+        {
+          "datasource": { "type": "prometheus", "uid": "prometheus" },
+          "expr": "sum by (name) (container_memory_usage_bytes{name=~\"galaxy-dev-.*|galaxy-game-.*\"})",
+          "legendFormat": "{{name}}"
+        }
+      ]
+    }
+  ],
+  "schemaVersion": 39,
+  "tags": ["galaxy"],
+  "templating": { "list": [] },
+  "time": { "from": "now-6h", "to": "now" },
+  "timepicker": {},
+  "title": "Galaxy — overview",
+  "uid": "galaxy-overview",
+  "version": 1,
+  "weekStart": ""
+}