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/prometheus/prometheus.yaml

@@ -0,0 +1,9 @@
+global:
+  scrape_interval: 30s
+  evaluation_interval: 30s
+
+scrape_configs:
+  - job_name: evcc
+    static_configs:
+      - targets: ['evcc:7070']
+    metrics_path: /metrics