fix(prices): unset VAT is 25% in the store and 0% in the planner forecast

[frahlg]

Unset VAT is 25% when prices are stored and 0% when the planner fills forecast slots. Overnight synthetic prices look cheaper than the published day-ahead and pull charging into the forecast tail.

Found on master 6eadec3f.

What happens

Harvester (go/internal/prices/prices.go):

vat := cfg.VATPercent
if vat == 0 {
    vat = 25
}

MPC wiring (go/cmd/ftw/main.go) copies raw cfg.Price.VATPercent (Go zero) into extendPricesWithForecast, which does total := (spot + gridTariff) * (1 + vatPct/100.0).

Published slots include 25% VAT. Forecast-extended slots do not. The planner sees a fake cheap tail after the day-ahead cutoff.

The same vat == 0 sentinel makes a real 0% VAT country impossible in the harvester while MPC would honor 0.

What a fix needs

• One VAT default, used by both the store and MPC (including extendPricesWithForecast).
• A distinct way to configure true 0% VAT (pointer, or a separate flag).
• Test: unset vat_percent, stored TotalOreKwh vs forecast-filled slot totals must use the same factor.

Open PR overlap: #1044 / #1051 touch prices. If either already unifies VAT, close this as duplicate of that PR; do not patch prices.go out from under them.