Auto-disable smart grid / require heartbeat msg to avoid being left in "high consumption modes" if control system fails

[lougreenwood]

I recently got the project working and I'm loving it - thank you to all contributors!

However I had an incident 2 days ago where the heat pump was turned on using the SG feature and then for some reason my control software which sends the SG control messages failed, so there was no message to power off when there was no solar surplus, leaving the system running in SG "forced on" during peak electricity prices.

As this incident shows, the control of SG is not robust or failure resistant enough and these failures can have financial consequences.

The idea I had was the addition of a SG mode where the control system must send a the espaltherma/sg/set message every X (30?) seconds - this will operate as a kind of "heartbeat ping" monitoring that the control system didn't go AWOL and the current SG mode is valid, functioning as a signal to ESPAltherma a signal that it should back out to it's normal running mode if the message isn't received.

If the message is not sent then it falls back to it's default state of "normal" SG mode.

This would be a breaking change, so probably wants to be a configurable option vs the current implementation.

Or maybe more generically we may just add an SG_MODE_MAX_TIME - type const, if it's undefined then there's no timeout. If a timeout is defined then the SG reverts to normal. If a new msg is received it resets the timer.

[lougreenwood]

I don't know C, i'm a typescript dev, but I might see how far I can get with Claude of o1-mini creating a PR for this over the next few days...

[lougreenwood]

FYI, I have a working version. Need to clean up a little (& add tests?) and will be able to publish a PR soon.