image pinning: eliminate :latest from all 5 unpinned templates + standing gate

bentopdf :latest -> v2.8.6; calibre-web :latest -> v4.0.6 (== running digest on
demo 9201, c31a738b - pin is a no-op); papra :latest -> 26.6.1-rootless (latest
was the rootless variant); recipe-importer :latest -> v0.9.11 (tag pre-existed,
digest-equal, no retag needed); termix :latest -> 2.5.0.

All five pins digest-identical to what :latest resolved to on 2026-07-12.
New gate scripts/check-image-pins.py (catches floating tags AND untagged refs;
red-proofed both shapes). Standing rule in CLAUDE.md + REUSE.md row.

templates/recipe-importer/docker-compose.yml

@@ -12,7 +12,7 @@
 
 services:
   recipe-importer:
-    image: gitea.dooplex.hu/admin/recipe-importer:latest
+    image: gitea.dooplex.hu/admin/recipe-importer:v0.9.11
     container_name: recipe-importer
     restart: unless-stopped
     environment: