fix(backup): exclude snap unit files from systemd rsync

Excludes snap-generated mount files (`*snap*.mount`) and `snapd.mounts.target.wants`
from the `/etc/systemd/system` rsync backup task.

Prevents 'Invalid argument (22)' errors caused by escaped backslashes (`\x2d`)
in dynamic snap mount file names when syncing to non-POSIX destination filesystems.

host_backup.sh

@@ -103,7 +103,10 @@ fi
 # Task 4: Backup Custom Systemd Units & Services
 # ==============================================================================
 if [ -d "$SYSTEMD_SOURCE" ]; then
-    rsync -aL "$SYSTEMD_SOURCE/" "$CURRENT_BACKUP_DIR/systemd/"
+    rsync -aL \
+        --exclude='*snap*.mount' \
+        --exclude='snapd.mounts.target.wants' \
+        "$SYSTEMD_SOURCE/" "$CURRENT_BACKUP_DIR/systemd/"
     echo "Backed up: $SYSTEMD_SOURCE"
 else
     echo "Warning: Systemd directory '$SYSTEMD_SOURCE' not found." >&2