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>

config.go

@@ -6,10 +6,15 @@ import (
 )
 
 type Config struct {
-	SampleRate int         `json:"sample_rate"` // seconds
-	AlphaEss   AlphaConf  `json:"alphaess"`
-	MQTT       MQTTConf   `json:"mqtt"`
-	DB         DBConf     `json:"db"`
+	SampleRate int           `json:"sample_rate"` // seconds
+	AlphaEss   AlphaConf    `json:"alphaess"`
+	MQTT       MQTTConf     `json:"mqtt"`
+	Charger    ChargerConf  `json:"charger"`
+	DB         DBConf       `json:"db"`
+}
+
+type ChargerConf struct {
+	Host string `json:"host"` // hostname or IP of go-e charger
 }
 
 type AlphaConf struct {