feat(security): add auditd/journald/logrotate role

Port Modules/Security/secharden-audit-agents.sh.

Deploys hardened configurations for:
- auditd (log rotation, disk space actions, email alerts)
- systemd-journald (persistent storage)
- logrotate (weekly rotation, 4 weeks retention)

This role is disabled by default (run_security_audit: false) matching
the legacy script which was commented out of the run order.

🤖 Generated with [Crush](https://github.com/charmassociates/crush)

Assisted-by: GLM-5 via Crush <crush@charm.land>

roles/security_audit/files/logrotate.conf

@@ -0,0 +1,23 @@
+# see "man logrotate" for details
+
+# global options do not affect preceding include directives
+
+# rotate log files weekly
+weekly
+
+# keep 4 weeks worth of backlogs
+rotate 4
+
+# create new (empty) log files after rotating old ones
+create 0640 root utmp
+
+# use date as a suffix of the rotated file
+#dateext
+
+# uncomment this if you want your log files compressed
+#compress
+
+# packages drop log rotation information into this directory
+include /etc/logrotate.d
+
+# system-specific logs may also be configured here.