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/doc/TODO

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+Here are some things that still have to be done. I add my thoughts
+
+about them, too. So you have an idea how likely you will see them
+
+in the next version. 
+
+If you want to do some of them *please* do so! I'm drowning in work.
+
+
+
+This file is always slightly out of date - to have full insight in
+
+the ongoing development process join the mailing list!
+
+
+
+* Template-Creation HOWTO
+
+  - which variables are available in which template and so on
+
+
+
+* Update/Add language files
+
+  - Yes please do so! You may even correct my bad english!
+
+
+
+* Export-Function
+