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/install/upgrade_v20.php

@@ -0,0 +1,35 @@
+<?php
+/**
+ * Database conversion script for DB v20
+ *
+ * @package Setup
+ * @author  Andreas Goetz <cpuidle@gmx.net>
+ * @version $Id: upgrade_v20.php,v 1.1 2007/01/04 16:14:16 andig2 Exp $
+ */
+
+
+/*
+ * Userseen data migration
+ */
+
+$sql	= 'SELECT owner_id, seen
+			 FROM videodata 
+			WHERE seen > 0';
+$set	= runSQL($sql, $dbh, true);
+if ($set === false) return(false);
+
+foreach ($set as $row)
+{
+    // don't convert fishy data
+    if (!empty($row['user_id']))
+    {
+        $sql = "REPLACE INTO ".TBL_USERSEEN." SET user_id=".$row['owner_id'].", video_id=".$row['id'];
+
+        if (runSQL($sql, $dbh) === false) return(false);
+    }        
+}
+
+// signal success
+return true;
+
+?>