fix(smtp_relay): emit service=smtp_relay in syslog so ingester can gate probe publish

SERVICE_NAME was hardcoded to 'smtp' in server.py; the ingester's probe
publish guard checked service == 'smtp_relay' and never matched.

Read SMTP_SERVICE_NAME from env (default 'smtp'); smtp_relay compose
fragment sets it to 'smtp_relay' so the two services are distinguishable.

decnet/services/smtp_relay.py

@@ -91,6 +91,7 @@ class SMTPRelayService(BaseService):
             "cap_add": ["NET_BIND_SERVICE"],
             "environment": {
                 "NODE_NAME": decky_name,
+                "SMTP_SERVICE_NAME": "smtp_relay",
                 "SMTP_OPEN_RELAY": "1",
                 "SMTP_QUARANTINE_DIR": _IN_CONTAINER_QUARANTINE,
             },