fix: update system now rebuilds docker image after git pull

- Add docker-compose package to Dockerfile (was missing, caused silent failure)
- Auto-detect current git branch instead of hardcoding 'main'
- Auto-select compose command (docker compose / docker-compose)
- Write update logs to persistent /app/logs/update.log volume
- Log git pull stdout/stderr for diagnostics

backend/Dockerfile

@@ -40,6 +40,7 @@ RUN apk add --no-cache \
     curl \
     git \
     docker-cli \
+    docker-compose \
     libstdc++ \
     tzdata \
     && mkdir -p /root/.ssh /app/logs \