fix: force the Zomboid branch switch with an explicit -beta public

Dropping -beta from app_update does not leave a beta branch. SteamCMD wrote
"public" into the manifest's UserConfig and left MountedConfig on "unstable",
so the install stayed on the 42.14.1 build from February while reporting
success. The config had already moved to the bare mod-ID syntax that 42.20+
wants, and 42.14.1 still requires the backslash prefix, so all 283 mods
loaded as "required mod ... not found" and the server came up vanilla.

Naming the branch explicitly is what actually remounts the content.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ansible/roles/podman/templates/zomboid/install.scmd.j2

@@ -13,7 +13,13 @@ force_install_dir /project-zomboid
 login anonymous
 
 // Install/Update the Project Zomboid Dedicated Server (Build 42, stable)
-// No -beta: B42 became the default branch at 42.20.
+//
-app_update 380870 validate
+// -beta public is required, not optional. Dropping the flag entirely does NOT
+// leave a previously-selected beta: SteamCMD writes the new branch to the
+// manifest's UserConfig but leaves MountedConfig -- and the actual installed
+// content -- on the old one. That failed silently here, leaving the server on
+// the 42.14.1 unstable build while the config had already moved to the mod-ID
+// syntax that only 42.20+ accepts, so every mod reported "not found".
+app_update 380870 -beta public validate
 
 quit