feat: add videodb media index with Docker stack

- Add videodb PHP/MySQL media collection manager (Blu-ray, DVD, CD)
- Dockerfile: PHP 8.1 + Apache with GD/mysqli/exif extensions
- docker-compose.yml: app on port 6761 + MySQL 8.0 with health checks
- docker-entrypoint.sh: auto-generates config.inc.php from env vars,
  waits for MySQL, initializes DB schema idempotently
- init-db.php: CLI schema installer using app's own prefix_query() logic
- Persistent volumes for DB, cache, and cover images

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

videodb/vendor/symfony/polyfill-php70/Resources/stubs/SessionUpdateTimestampHandlerInterface.php

@@ -0,0 +1,23 @@
+<?php
+
+interface SessionUpdateTimestampHandlerInterface
+{
+    /**
+     * Checks if a session identifier already exists or not.
+     *
+     * @param string $key
+     *
+     * @return bool
+     */
+    public function validateId($key);
+
+    /**
+     * Updates the timestamp of a session when its data didn't change.
+     *
+     * @param string $key
+     * @param string $val
+     *
+     * @return bool
+     */
+    public function updateTimestamp($key, $val);
+}