feat: shutdown node cleanly on signal

[agaffney]

Fixes #72

[agaffney]

$ ./dingo
{"time":"2025-04-07T17:03:38.834584146-04:00","level":"INFO","msg":"maxprocs: Leaving GOMAXPROCS=8: CPU quota undefined","component":"dingo"}
{"time":"2025-04-07T17:03:38.834625416-04:00","level":"INFO","msg":"version: devel (commit 5f9cf14)","component":"dingo"}
{"time":"2025-04-07T17:03:38.837575558-04:00","level":"INFO","msg":"serving prometheus metrics on 0.0.0.0:12798","component":"node"}
...
^C{"time":"2025-04-07T17:03:43.269910893-04:00","level":"INFO","msg":"signal received, shutting down"}