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>

db.go

@@ -50,6 +50,14 @@ func (db *DB) WriteMeter(ctx context.Context, t time.Time, r MeterReading) error
 	return err
 }
 
+func (db *DB) WriteCharger(ctx context.Context, r ChargerReading) error {
+	_, err := db.pool.Exec(ctx,
+		`INSERT INTO charger (time, power, eto_wh) VALUES ($1, $2, $3)`,
+		r.Time, r.Power, r.EtoWh,
+	)
+	return err
+}
+
 func (db *DB) Close() {
 	db.pool.Close()
 }