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.

dankserver/plex/quadlet/plex.container

@@ -4,7 +4,7 @@ After=network-online.target
 Wants=network-online.target
 
 [Container]
-Image=lscr.io/linuxserver/plex:1.43.5.11029-6bf8af3f0-ls329
+Image=lscr.io/linuxserver/plex:1.43.5.11029-6bf8af3f0-ls329@sha256:8f655bad7dcbc5398a78f70a5af12600b4ed3424f901566ed1eaa6710e9e2334
 ContainerName=plex
 Network=dankserver_appnet:alias=plex
 Environment=PUID=0