Fix WAL archive_command failing when segment already exists

pg_basebackup -Xs copies WAL segments to the backup dir, causing
the archive_command to fail on those pre-existing files and block
all subsequent archiving.

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

docker-compose.yml

@@ -21,7 +21,7 @@ services:
       - -c
       - archive_mode=on
       - -c
-      - archive_command=test ! -f /backup/wal/%f && cp %p /backup/wal/%f
+      - archive_command=test -f /backup/wal/%f || cp %p /backup/wal/%f
 
   postgres-backup:
     image: postgres:18