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.

shared/prometheus/prometheus.yml

@@ -131,3 +131,63 @@ scrape_configs:
       - targets: ["192.168.0.235:7770"]
         labels:
           service: frank-archon
+
+  # Per-container metrics from cadvisor (a rootless quadlet on each host).
+  # cadvisor keys every series by the cgroup path; for a quadlet container that
+  # path is …/app.slice/<name>.service[/libpod-payload-<hex>], so the unit NAME
+  # lives in it. The relabel below pulls that name out into a `container` label
+  # and keeps only the per-unit aggregate — cadvisor's raw host root,
+  # system.slice daemons and machine.slice scopes carry no such name and are
+  # dropped, as is the per-CPU-core split (only cpu="total" survives), so the
+  # scrape stays small. cadvisor cannot bind per-interface, so it is reached at
+  # the host's PUBLISHED LAN port: unlike node-exporter's host-net bind (which
+  # is why node-dankclaw scrapes via host.containers.internal), a published port
+  # IS reachable from this bridge container at the host LAN address.
+  - job_name: "cadvisor-dankclaw"
+    static_configs:
+      - targets: ["192.168.0.143:8080"]
+        labels:
+          host: dankclaw
+    metric_relabel_configs:
+      - source_labels: [id]
+        regex: '.*/app\.slice/([^.]+)\.service(?:/.*)?$'
+        target_label: container
+        replacement: '$1'
+      # Keep only the unit-level cgroup; drop its payload/control children so a
+      # container is not counted twice.
+      - source_labels: [id]
+        regex: '.*/\.service/.*'
+        action: drop
+      # Drop everything that is not a named unit (no `container` was extracted).
+      - source_labels: [container]
+        regex: '.+'
+        action: keep
+      # cpu="total" for cpu series; metrics with no cpu label (memory, network,
+      # filesystem) are kept.
+      - source_labels: [cpu]
+        regex: '^$|total'
+        action: keep
+      - regex: 'id|name|image|container_label.*|machine_id|boot_id'
+        action: labeldrop
+
+  - job_name: "cadvisor-dankserver"
+    static_configs:
+      - targets: ["192.168.0.235:8080"]
+        labels:
+          host: dankserver
+    metric_relabel_configs:
+      - source_labels: [id]
+        regex: '.*/app\.slice/([^.]+)\.service(?:/.*)?$'
+        target_label: container
+        replacement: '$1'
+      - source_labels: [id]
+        regex: '.*/\.service/.*'
+        action: drop
+      - source_labels: [container]
+        regex: '.+'
+        action: keep
+      - source_labels: [cpu]
+        regex: '^$|total'
+        action: keep
+      - regex: 'id|name|image|container_label.*|machine_id|boot_id'
+        action: labeldrop