fix(deploy): stop the panel from pointing at a tag the build dropped

USERBOT_IMAGE named userbot:latest, but the build stopped pushing latest when
images became :prod. render_pinned resolves every gcr.forust.xyz reference it
finds in a manifest, not just the ones it rewrites, so that env var made the
whole apply abort with "cannot resolve userbot:latest; applying nothing".

Naming it :prod also makes the build job rebuild userbot, which is what
restores forust/userbot:prod - the tag is listed in the registry but its
manifest 404s, so the image: field in userbots.yaml had nothing to resolve
either.

Found by resolving every digest apply-k8s will need before spending a run on
it: 6 of 8 resolved, and both failures were userbot.

userbot/k8s/base/panel.yaml

@@ -208,7 +208,9 @@ spec:
             - name: USERBOT_LEGACY_NAMESPACES
               value: default
             - name: USERBOT_IMAGE
-              value: gcr.forust.xyz/forust/userbot:latest
+              # The build pushes main/prod only. The deploy resolves every gcr ref in
+              # this file, so a :latest here aborts the whole apply as unresolvable.
+              value: gcr.forust.xyz/forust/userbot:prod
             - name: USERBOT_STORAGE_CLASS
               value: local-path-retain
             - name: USERBOT_DOWNLOADS_HOST_PATH