feat(security): digest-pin every registry image on all three hosts (fleet law)

Closing the command-center security queue #70/#71/#72/#73/#74: rolling tags
let a rebuild move under a running service with nobody approving it, and
make a CVE audit ungradeable. 36 quadlet units across dankclaw, dankserver
and frankbook-pro now carry tag@sha256; localhost/* builds stay exempt
(their build is the pin). The pihole and copyparty pins pull the fixed
builds; valkey 9.1.2 (already running) and redis 7.4.11 are pinned in
place, vendor fixes TBD.

New gate tests/quadlet_image_pins.bats: no unpinned registry image, no
generator-rejected key anywhere. Three seams that pinned the old exact-tag
law (frankbook readarr, open_webui, seerr) now require tag AND digest -
strictly stronger. Quadlet Lifecycle docs: batch-abort law, entrypoint
flags-only law, pin law.

dankclaw/observability/quadlet/alertmanager.container

@@ -33,7 +33,7 @@ Wants=network-online.target
 
 [Container]
 ContainerName=alertmanager
-Image=docker.io/prom/alertmanager:latest
+Image=docker.io/prom/alertmanager:latest@sha256:e9733bafb1bdef9b00e25a21f8f99dc26a22224bf16641ad754d1649f4c3357a
 Network=dankclaw_default
 User=0:0
 Exec=--config.file=/etc/alertmanager/alertmanager.yml --storage.path=/alertmanager

dankclaw/observability/quadlet/cadvisor.container

@@ -35,7 +35,7 @@ Wants=network-online.target
 
 [Container]
 ContainerName=cadvisor
-Image=gcr.io/cadvisor/cadvisor:v0.49.1
+Image=gcr.io/cadvisor/cadvisor:v0.49.1@sha256:3cde6faf0791ebf7b41d6f8ae7145466fed712ea6f252c935294d2608b1af388
 PodmanArgs=--pid=host --cgroupns=host
 PublishPort=127.0.0.1:8080:8080
 PublishPort=192.168.0.143:8080:8080

dankclaw/observability/quadlet/grafana.container

@@ -34,7 +34,7 @@ Wants=network-online.target prometheus.service
 
 [Container]
 ContainerName=grafana
-Image=docker.io/grafana/grafana:latest
+Image=docker.io/grafana/grafana:latest@sha256:dbbc38ea21103cefb50396340cd88afec779c7e070b0a7bf5a75b4ce46651d3f
 Network=dankclaw_default
 User=0:0
 PublishPort=127.0.0.1:3002:3000

dankclaw/observability/quadlet/node-exporter.container

@@ -33,7 +33,7 @@ Wants=network-online.target
 
 [Container]
 ContainerName=node-exporter
-Image=docker.io/prom/node-exporter:latest
+Image=docker.io/prom/node-exporter:latest@sha256:1b4e4438faca4dd7e001dd445d161a4a2091b0fededa84093b3a8dfeae1f1be0
 Network=host
 PodmanArgs=--pid=host
 Exec=--path.procfs=/host/proc --path.sysfs=/host/sys --path.rootfs=/rootfs --web.listen-address=127.0.0.1:9100 --web.listen-address=192.168.0.143:9100