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.

tests/open_webui_quadlet.bats

@@ -15,8 +15,9 @@ QDIR="$REPO_ROOT/dankclaw/open-webui/quadlet"
 }
 
 @test "open-webui uses the CPU image, not :cuda (dankclaw is CPU-only)" {
-    grep -qE '^Image=ghcr\.io/open-webui/open-webui:main$' "$QDIR/open-webui.container" \
-        || { echo "open-webui.container must use the :main (CPU) image"; return 1; }
+    # 2026-10-21 fleet law: tag AND digest (tests/quadlet_image_pins.bats)
+    grep -qE '^Image=ghcr\.io/open-webui/open-webui:main@sha256:[0-9a-f]{64}$' "$QDIR/open-webui.container" \
+        || { echo "open-webui.container must use the :main (CPU) image, digest-pinned"; return 1; }
     if grep -qE '^Image=.*:cuda' "$QDIR/open-webui.container"; then
         echo "open-webui.container must not use the :cuda image on a CPU-only host"
         return 1