let's call this key `heartbeat_interval` and make it an `int` (this way we don't need to do the `strconv` below).  Because we're making it an `int` and Go has this concept of default values it means we can't use `0` to mean "turn off heartbeats" (if someone did not specify the `heartbeat_interval` key then it would default to 0 and inadvertently disable heartbeats).

We should also switch from seconds to milliseconds.

With that, the rules could be: `-1 = disable, 0 = default, 100ms <= heartbeat <= 60000ms`.  Any other value is invalid.  I'm not sure I see much reason for smaller or higher intervals.