left4me: assert nftables + systemd bundle membership

Catches misconfiguration at bw test time if a node attaches left4me
without those two bundles. Both contribute load-bearing metadata
materializers (nftables/output rules; systemd/units → unit files).

bundles/left4me/metadata.py

@@ -1,3 +1,11 @@
+assert node.has_bundle('nftables'), (
+    f'{node.name}: bundle left4me requires bundle nftables (uses nftables/output for srcds DSCP marking)'
+)
+assert node.has_bundle('systemd'), (
+    f'{node.name}: bundle left4me requires bundle systemd (declares units via systemd/units metadata)'
+)
+
+
 defaults = {
     'left4me': {
         'gunicorn_workers': 1,