feat(infra) : add maintenance mode during deployments

Nginx returns a 503 page when maintenance.on exists. The deploy script
automatically enables/disables maintenance mode around the update.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

infra/prod/deploy.sh

@@ -8,6 +8,9 @@ export LESSTIME_IMAGE_TAG="$TAG"
 
 echo "==> Deploying lesstime:${TAG}..."
 
+echo "==> Enabling maintenance mode..."
+touch maintenance.on
+
 echo "==> Pulling image..."
 sudo docker compose pull
 
@@ -24,5 +27,8 @@ echo "==> Clearing cache..."
 sudo docker compose exec -T -u www-data app php bin/console cache:clear --env=prod
 sudo docker compose exec -T -u www-data app php bin/console cache:warmup --env=prod
 
+echo "==> Disabling maintenance mode..."
+rm -f maintenance.on
+
 VERSION=$(sudo docker compose exec -T app cat config/version.yaml | grep 'app.version' | awk -F"'" '{print $2}')
 echo "==> Deployed v${VERSION}"