fix(deploy): roll out our images by digest instead of a moving tag

`kubectl rollout undo` restores the previous ReplicaSet's pod template
verbatim. While that template names a tag, the rollback does not roll back
the image: the tag has already moved, so the reverted pod pulls the very
build that just failed and the cluster stays broken. The safety net added
in 1505b63 therefore could not recover from a bad image.

Pin the digest at apply time. A digest is not knowable when a manifest is
written, so render_pinned resolves it on the way into the cluster and the
digest is never committed. Git keeps a readable `:prod`, Renovate keeps
seeing exactly the manifests it saw before, and the previous revision of
each workload now holds the digest that was actually serving, so undo
restores those exact bytes.

imagePullPolicy is dropped from the manifests rather than set to
IfNotPresent: a reference that is not `:latest` already defaults to it, and
that is what the Kubernetes docs ask for alongside a digest.

An unresolvable image is fatal instead of a warning, because carrying on
would quietly apply a mutable tag again.

restart_stale_images keeps its comparison but is no longer how a rebuild
reaches the cluster -- the pinned template rolls out on its own now. What
is left is a drift check for hand-run `kubectl set image`, so it matches
the container by repository: a pod's status now reports `repo@sha256:...`
while the manifest still says `:prod`.

The build job stops pushing `:latest` altogether, which removes the tag
that a dev branch could otherwise move under a prod deploy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -361,7 +361,7 @@ jobs:
             case "$service" in
               dtek_notif)
                 image="${REGISTRY}/forust/dtek-notif"
-                tags=("latest")
+                tags=()
                 case "${GITHUB_REF_NAME}" in
                   main)
                     tags+=("main" "prod")
@@ -384,7 +384,7 @@ jobs:
                 ;;
               errorpages)
                 image="${REGISTRY}/forust/error-pages"
-                tags=("latest")
+                tags=()
                 case "${GITHUB_REF_NAME}" in
                   main)
                     tags+=("main" "prod")
@@ -406,7 +406,7 @@ jobs:
                 done
                 ;;
               userbot)
-                tags=("latest")
+                tags=()
                 case "${GITHUB_REF_NAME}" in
                   main)
                     tags+=("main" "prod")
@@ -449,7 +449,7 @@ jobs:
                       image="${REGISTRY}/forust/xdfnx-homepage"
                       ;;
                   esac
-                  tags=("latest")
+                  tags=()
                   case "${GITHUB_REF_NAME}" in
                     main)
                       tags+=("main" "prod")
@@ -483,7 +483,7 @@ jobs:
                       image="${REGISTRY}/forust/webinar-checker"
                       ;;
                   esac
-                  tags=("latest")
+                  tags=()
                   case "${GITHUB_REF_NAME}" in
                     main)
                       tags+=("main" "prod")