stalwart: pin to v0.16.27 until v1 migration is supported

Stalwart v1.0.0 (26 Oct 2026) ships a new data store layout, so v0.16.x
deployments cannot be upgraded in place. Data has to be copied into a new,
empty v1 store with stalwartlabs/stalwart-migrate, which requires v0.16.27
as its source version and needs an interactive prepare step plus an offline
final cutover.

The update path pulled "latest" and would have swapped the binary for v1 on
existing containers, leaving the mail server unable to start on its old store.
Pin install and update to v0.16.27 instead and surface the reason through
check_for_gh_release. Containers that already pulled v1.0.0 are put back on
v0.16.27 by the next update run.

ct/stalwart.sh

@@ -31,12 +31,13 @@ function update_script() {
     exit
   fi
 
-  if check_for_gh_release "stalwart" "stalwartlabs/stalwart"; then
+  RELEASE="v0.16.27"
+  if check_for_gh_release "stalwart" "stalwartlabs/stalwart" "${RELEASE}" "Stalwart v1 changes the data store layout and requires an offline migration with stalwartlabs/stalwart-migrate; pinned until this script supports it"; then
     msg_info "Stopping Service"
     systemctl stop stalwart
     msg_ok "Stopped Service"
 
-    CLEAN_INSTALL=1 fetch_and_deploy_gh_release "stalwart" "stalwartlabs/stalwart" "prebuild" "latest" "/opt/stalwart" "stalwart-$(arch_resolve x86_64 aarch64)-unknown-linux-gnu.tar.gz"
+    CLEAN_INSTALL=1 fetch_and_deploy_gh_release "stalwart" "stalwartlabs/stalwart" "prebuild" "${RELEASE}" "/opt/stalwart" "stalwart-$(arch_resolve x86_64 aarch64)-unknown-linux-gnu.tar.gz"
     chmod +x /opt/stalwart/stalwart
 
     msg_info "Starting Service"

install/stalwart-install.sh

@@ -13,7 +13,7 @@ setting_up_container
 network_check
 update_os
 
-fetch_and_deploy_gh_release "stalwart" "stalwartlabs/stalwart" "prebuild" "latest" "/opt/stalwart" "stalwart-$(arch_resolve x86_64 aarch64)-unknown-linux-gnu.tar.gz"
+fetch_and_deploy_gh_release "stalwart" "stalwartlabs/stalwart" "prebuild" "v0.16.27" "/opt/stalwart" "stalwart-$(arch_resolve x86_64 aarch64)-unknown-linux-gnu.tar.gz"
 chmod +x /opt/stalwart/stalwart
 
 msg_info "Configuring Stalwart"