Implement Bodenfeuchte app: MQTT listener, SQLite storage, chart UI, Docker

- Custom Next.js server starts MQTT listener on boot
- Subscribes to zigbee2mqtt/Bodenfeuchte_1, stores soil_moisture in SQLite
- API route /api/data returns last 6 hours of measurements
- Frontend shows current value + Recharts line chart, auto-refresh every 60s
- Dockerfile + docker-compose with persistent volume for SQLite DB

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

.gitignore

@@ -35,6 +35,12 @@ yarn-error.log*
 .env.local
 .env.*.local
 
+# sqlite data
+/data/*.db
+
+# compiled server
+/dist
+
 # vercel
 .vercel