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/tempo/tempo.yml

@@ -0,0 +1,30 @@
+# Single-binary Tempo for the dev stack: OTLP receivers, local block
+# storage, 3-day retention. Internal-only (no host port). Backend and
+# gateway push traces here over OTLP gRPC (4317).
+server:
+  http_listen_port: 3200
+  log_level: warn
+
+distributor:
+  receivers:
+    otlp:
+      protocols:
+        grpc:
+          endpoint: 0.0.0.0:4317
+        http:
+          endpoint: 0.0.0.0:4318
+
+ingester:
+  max_block_duration: 5m
+
+compactor:
+  compaction:
+    block_retention: 72h
+
+storage:
+  trace:
+    backend: local
+    local:
+      path: /var/tempo/blocks
+    wal:
+      path: /var/tempo/wal