fix: use /usr/bin/mariadb-admin for mariadb healthcheck

MariaDB 11 renamed mysqladmin → mariadb-admin; neither the old
healthcheck.sh probe nor mysqladmin is reachable from the Docker
healthcheck exec context. Switch to the full-path binary that is
confirmed present in the container.

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

docker-compose.yml

@@ -42,7 +42,7 @@ services:
       # silently no-op on Debian/Ubuntu and macOS Docker Desktop.
       - ./mariadb/init:/docker-entrypoint-initdb.d:ro,Z
     healthcheck:
-      test: ["CMD", "mysqladmin", "ping", "-h", "127.0.0.1", "--silent"]
+      test: ["CMD", "/usr/bin/mariadb-admin", "ping", "-h", "127.0.0.1", "--silent"]
       interval: 10s
       timeout: 5s
       retries: 12