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/open-webui/quadlet/open-webui.container

@@ -27,7 +27,7 @@ After=network.target
 [Container]
 ContainerName=open-webui
 Network=open-webui.network
-Image=ghcr.io/open-webui/open-webui:main
+Image=ghcr.io/open-webui/open-webui:main@sha256:4860ba005ae8f19f08d1e12b0868855b7b339ea0dad9927f196820fc0de04575
 PublishPort=3005:8080
 Environment=OPENAI_API_BASE_URL=http://192.168.0.143:7770
 Environment=ANONYMIZED_TELEMETRY=False