Fix WAL archive permission denied by chowning backup dir to postgres UID

The backup container creates /backup/wal as root, but the postgres
archive_command runs as UID 999 and can't write to it.

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

docker-compose.yml

@@ -38,6 +38,7 @@ services:
     command:
       - |
         mkdir -p /backup/wal /backup/full
+        chown 999:999 /backup/wal
         while true; do
           STAMP=$$(date +%Y-%m-%d_%H-%M-%S)
           echo "Starting full backup: full_$$STAMP"