use journal namespace

config/journald/journald@terraria.conf

@@ -0,0 +1,11 @@
+# Settings for the Terraria servers' own journal (LogNamespace=terraria).
+# Read it with:  journalctl --namespace=terraria -u terraria@<name>
+# After changing this file:  sudo systemctl restart systemd-journald@terraria
+# All options: man journald.conf
+
+[Journal]
+# Disk space the servers' logs may use, kept separate from the system journal
+SystemMaxUse=256M
+RuntimeMaxUse=64M
+# Uncomment to delete entries older than this
+#MaxRetentionSec=3month

config/tmpfiles.d/terractl.conf

@@ -3,3 +3,10 @@ d /var/lib/terraria            0750 terraria terraria -
 d /var/lib/terraria/instances  0750 terraria terraria -
 # setgid + group-writable: members of the 'terraria' group can upload zips
 d /var/lib/terraria/dist       2775 root     terraria -
+
+# The servers' journal (LogNamespace=terraria): readable by the terraria group,
+# including files journald creates later (default ACL). 'A+' only acts on paths
+# that exist and never creates them; 'terractl journal-access' creates the
+# folders, and only where journald already keeps journals.
+A+ /var/log/journal/%m.terraria - - - - group:terraria:r-x,default:group:terraria:r-x
+A+ /run/log/journal/%m.terraria - - - - group:terraria:r-x,default:group:terraria:r-x