Question here: is there a generic way to communicate fixed feed-in curtailment which is implemented on inverter side so that optimizer can use it? This hems piece seems more relevant for steuerbox cases, can't find anything in documentation.

Question here

That's a different topic, lets not hide it here.

Code review

Bug: missing nil-check on site.hems will break the optimizer for any site without a HEMS configured

// soft grid feed-in cap from active HEMS curtailment (e.g. German 70% rule):
// export is capped at this power, excess PV is curtailed instead of exported
if curtailed := hems.Curtailed(site.hems); curtailed != nil && *curtailed {
    if pMaxExp := site.hems.MaxProductionPower(); pMaxExp != nil {
        req.Grid.PMaxExp = float32(*pMaxExp)
    }
}

site.hems is a nil api.HEMS interface whenever no HEMS device is configured (it's only ever set via site.SetHEMS, which cmd/setup.go only calls when a HEMS device actually exists). hems.Curtailed calls hems.CurtailedPercent() on the argument unconditionally with no nil guard:

func Curtailed(hems api.HEMS) *bool {
	percent := hems.CurtailedPercent() // panics on a nil interface
	...
}

Calling a method on a nil interface value panics. Since optimizerUpdate runs on every optimizer cycle regardless of whether HEMS is configured, this panics every cycle for any installation that uses the optimizer without a HEMS device — the panic is caught by the recover() in optimizerUpdateAsync, so it doesn't crash the process, but it leaves the optimizer feature permanently non-functional for that large segment of users (logged as an error every cycle instead).

The existing sibling code just above (if site.circuit != nil { ... }) and the established pattern elsewhere in the codebase (e.g. core/site.go guards every use of site.hems with if site.hems != nil) both show the intended guard was simply omitted here.

Suggested fix:

if site.hems != nil {
    if curtailed := hems.Curtailed(site.hems); curtailed != nil && *curtailed {
        if pMaxExp := site.hems.MaxProductionPower(); pMaxExp != nil {
            req.Grid.PMaxExp = float32(*pMaxExp)
        }
    }
}