feat(prices): every European bidding zone, billed in its own currency

.changeset/european-price-zones.md

@@ -0,0 +1,40 @@
+---
+"ftw": minor
+---
+
+Prices: pick any of the 46 European bidding zones the price API publishes,
+and be billed in the currency of the country you picked.
+
+The zone picker offered twelve Nordic codes typed into two `<select>`
+elements and the Go side knew twelve EIC codes, so a household in Belgium,
+the Netherlands or Spain could not choose its own zone even though the
+Sourceful API has served every ENTSO-E area all along. Both lists now come
+from one table, `go/internal/prices/zones.go`, generated from that API's
+`/areas` endpoint and served to the UI at `GET /api/prices/zones`. The
+picker asks for a country first and a zone second, because everyone knows
+they live in Italy and nobody knows their area code is `IT-CENTRE-NORTH`.
+
+Currency stops being Swedish by assumption. It defaults to the currency of
+the chosen zone, and the price API — which converts to EUR and SEK and
+quietly answers anything else with EUR — is only asked for those two;
+every other currency is converted here from EUR with the ECB rates the
+service already caches. Where no rate is available the fetch fails rather
+than storing a number that is wrong by an exchange rate, which is a number
+the planner would spend money on. The old 11.5 SEK/EUR fallback is gone for
+the same reason.
+
+Two related faults fixed on the way. The direct ENTSO-E provider assumed
+every day-ahead document is priced in EUR; Poland and Hungary among others
+publish in their own currency, so it now reads `currency_Unit.name` and
+converts from that. Its EIC code for Germany was the country code rather
+than the DE-LU bidding zone, and NO5 was missing outright.
+
+Prices are stored as minor units per kWh with no currency attached, so
+changing the currency clears the price cache — otherwise cost history would
+add öre to cent. The next fetch refills today and tomorrow. Every price
+label in the UI now follows the configured currency: öre, cent, øre, grosz,
+Rappen, or the major unit where the minor one is out of circulation
+(4.30 Kč/kWh, not 430 haléř).
+
+Existing installs are untouched: no zone means SE3, no currency means SEK,
+and a Swedish install still asks the API for SEK exactly as before.

go/internal/api/api.go

@@ -307,6 +307,7 @@ func (s *Server) routes() {
 	s.handle("GET  /api/energy/history.csv", s.handleEnergyHistoryCSV)
 	s.handle("GET  /api/savings/daily", s.handleSavingsDaily)
 	s.handle("GET  /api/prices", s.handlePrices)
+	s.handle("GET  /api/prices/zones", s.handlePriceZones)
 	s.handle("GET  /api/forecast", s.handleForecast)
 	s.handle("GET  /api/mpc/plan", s.handleMPCPlan)
 	s.handle("POST /api/mpc/replan", s.handleMPCReplan)
@@ -1989,12 +1990,34 @@ func (s *Server) handlePrices(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	writeJSON(w, 200, map[string]any{
-		"zone":    s.deps.Prices.Zone,
-		"items":   rows,
-		"enabled": true,
+		"zone":     s.deps.Prices.Zone,
+		"currency": s.deps.Prices.Currency,
+		"items":    rows,
+		"enabled":  true,
 	})
 }
 
+// ---- /api/prices/zones ----
+//
+// The bidding zones a household can pick, with the currency each one is
+// billed in. Served from the same table the fetchers use, so the picker
+// can't offer a zone the providers don't know.
+//
+// Response: {"zones": [{code, country, currency, name}]}
+func (s *Server) handlePriceZones(w http.ResponseWriter, _ *http.Request) {
+	all := prices.Zones()
+	items := make([]map[string]string, 0, len(all))
+	for _, z := range all {
+		items = append(items, map[string]string{
+			"code":     z.Code,
+			"country":  z.Country,
+			"currency": z.Currency,
+			"name":     z.Name(),
+		})
+	}
+	writeJSON(w, 200, map[string]any{"zones": items})
+}
+
 // ---- /api/forecast ----
 //
 // Query params: range=24h|48h|3d (default 48h lookahead).