feat(observability): per-container metrics via cadvisor + Containers dashboard

Per-container CPU/memory was invisible for the quadlet app stack — Grafana could
only see the host (node_exporter) and the LLM engines. This adds a rootless
cadvisor quadlet on each host and a `containers` dashboard.

cadvisor works on this rootless-Podman / cgroup-v2 host on one condition:
--cgroupns=host. Podman starts every container in a private cgroup namespace, so
without it cadvisor sees only its own cgroup root and exports a single series
(measured 2026-10-20: 1 series off, 996 on). The libpod socket does not name the
rootless app containers (it resolved 2 of 34), so naming comes from elsewhere:
the quadlet unit name is carried in the cgroup path
(app.slice/<name>.service/libpod-payload-<hex>), and Prometheus relabels it into
a `container` label. No podman socket needed.

cadvisor cannot bind per-interface like node_exporter, so each unit is a bridge
container publishing exactly 127.0.0.1:8080 and the host LAN address — never a
wildcard. Prometheus (a bridge container) reaches a host-published port at the
LAN address, so cadvisor-dankclaw/dankserver scrape 192.168.0.143:8080 /
192.168.0.235:8080. Only the families cadvisor actually exports for these
containers are graphed — CPU usage, memory working-set/limit, OOM — not network
or filesystem, which are absent under the host-PID layout.

Wired through the deployer (allowlist, deploy_observability case + /healthz gate,
dispatcher), the planner (both targets), and observability_quadlet.bats.

dankserver/observability/quadlet/cadvisor.container

@@ -0,0 +1,39 @@
+# Quadlet unit: cadvisor (rootless, boot-persistent) — DANKSERVER.
+#
+# Install path on dankserver: ~/.config/containers/systemd/cadvisor.container
+# Unit name: cadvisor.service (container name: cadvisor)
+#
+# The Dankserver twin of dankclaw/observability/quadlet/cadvisor.container.
+# Same rootless shape, same --cgroupns=host requirement (see that file for the
+# full story), only the LAN address differs: this host publishes on
+# 192.168.0.235:8080 so Prometheus on Dankclaw can scrape it over the real
+# network (cross-host, so the host.containers.internal trick does not apply).
+# It reports the media/library stack (romm, plex, jellyfin, copyparty, calibre,
+# tautulli) by their quadlet unit names via the cgroup path.
+
+[Unit]
+Description=cAdvisor container metrics (Dankserver)
+After=network-online.target
+Wants=network-online.target
+
+[Container]
+ContainerName=cadvisor
+Image=gcr.io/cadvisor/cadvisor:v0.49.1
+PodmanArgs=--pid=host --cgroupns=host
+PublishPort=127.0.0.1:8080:8080
+PublishPort=192.168.0.235:8080:8080
+Volume=/sys:/sys:ro
+Volume=/:/rootfs:ro
+Exec=--housekeeping_interval=30s
+HealthCmd=wget --spider -q http://localhost:8080/healthz
+HealthInterval=30s
+HealthTimeout=10s
+HealthRetries=3
+HealthStartPeriod=40s
+
+[Service]
+Restart=always
+RestartSec=10
+
+[Install]
+WantedBy=default.target