fix: replace mysqladmin with PHP mysqlnd for MySQL 8 compatibility

The MariaDB CLI client (default-mysql-client on Debian) cannot complete
the caching_sha2_password handshake used by MySQL 8.0, so the entrypoint
wait loop never exited.

- Dockerfile: remove default-mysql-client (no longer needed)
- docker-entrypoint.sh: wait loop now uses a PHP one-liner via mysqli_connect
  (same mysqlnd driver the app uses — if it connects, the app will too)
- docker-compose.yml: health check pings via local socket without credentials
  (mysqladmin ping -h localhost --silent works inside the DB container)

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

videodb/Dockerfile

@@ -6,7 +6,6 @@ RUN apt-get update && apt-get install -y \
     libjpeg-dev \
     libfreetype6-dev \
     libonig-dev \
-    default-mysql-client \
     && rm -rf /var/lib/apt/lists/*
 
 # Install PHP extensions needed by videoDB