Add go-e wallbox charger integration

- charger.go: polls go-e /api/status?filter=nrg,eto every 10 s
- db.go: WriteCharger() inserts into charger hypertable
- config.go: ChargerConf with host field
- main.go: polls charger in parallel with inverter and meters
- schema.sql: charger table + charger_10m/1h/daily aggregates + policies

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

charger.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+)
+
+type ChargerReading struct {
+	Time  time.Time
+	Power float32 // W total
+	EtoWh int64   // Wh cumulative
+}
+
+type chargerStatus struct {
+	Nrg []float64 `json:"nrg"` // [0..15] voltages/currents/powers; [11] = total W
+	Eto int64     `json:"eto"` // cumulative Wh
+}
+
+var chargerClient = &http.Client{Timeout: 5 * time.Second}
+
+func PollCharger(host string) (ChargerReading, error) {
+	url := "http://" + host + "/api/status?filter=nrg,eto"
+	resp, err := chargerClient.Get(url)
+	if err != nil {
+		return ChargerReading{}, fmt.Errorf("get: %w", err)
+	}
+	defer resp.Body.Close()
+
+	var s chargerStatus
+	if err := json.NewDecoder(resp.Body).Decode(&s); err != nil {
+		return ChargerReading{}, fmt.Errorf("decode: %w", err)
+	}
+	if len(s.Nrg) < 12 {
+		return ChargerReading{}, fmt.Errorf("nrg array too short (%d)", len(s.Nrg))
+	}
+	return ChargerReading{
+		Time:  time.Now(),
+		Power: float32(s.Nrg[11]),
+		EtoWh: s.Eto,
+	}, nil
+}