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>

docker-compose.yml

@@ -35,7 +35,7 @@ services:
     # MySQL 8 defaults to caching_sha2_password; keep native auth for broad client compat
     command: --default-authentication-plugin=mysql_native_password
     healthcheck:
-      test: ["CMD", "mysqladmin", "ping", "-h", "localhost", "-u", "videodb", "-pvideodb_secret"]
+      test: ["CMD", "mysqladmin", "ping", "-h", "localhost", "--silent"]
       interval: 10s
       timeout: 5s
       retries: 12