fix: use kill -0 1 for litestream healthcheck

pgrep may not be available in the litestream image. kill -0 1 checks
whether PID 1 (litestream, after exec) is alive — works in any container.

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

docker-compose.prod.yml

@@ -35,7 +35,7 @@ services:
       - app-data:/app/data
       - ./padelnomics/litestream.yml:/etc/litestream.yml:ro
     healthcheck:
-      test: ["CMD-SHELL", "pgrep -x litestream > /dev/null"]
+      test: ["CMD-SHELL", "kill -0 1"]
       interval: 5s
       timeout: 5s
       retries: 6