Add Docker Compose stack for son's installation (evcc + Prometheus + Grafana)

Self-contained stack for DS218 Synology targeting:
- 2x Fronius inverters (11+22 kW) via shared DataManager, Solar.API v1
- BYD 12 kWh battery connected through Fronius
- 2x Elli Charger Connect 2 via OCPP 1.6j
- evcc handles PV surplus charging, OCPP server built-in
- Prometheus scrapes evcc /metrics, 90d retention
- Grafana auto-provisioned with evcc dashboard and Prometheus datasource

Setup: edit .env (3 IPs + 2 station IDs), point Elli app to OCPP backend,
docker compose up -d. README in German for non-IT handover.

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

deploy/son/grafana/provisioning/dashboards/dashboard.yaml

@@ -0,0 +1,10 @@
+apiVersion: 1
+
+providers:
+  - name: evcc
+    type: file
+    disableDeletion: false
+    updateIntervalSeconds: 60
+    options:
+      path: /etc/grafana/provisioning/dashboards
+      foldersFromFilesStructure: false

deploy/son/grafana/provisioning/datasources/prometheus.yaml

@@ -0,0 +1,9 @@
+apiVersion: 1
+
+datasources:
+  - name: Prometheus
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    isDefault: true
+    editable: false