Remove unused backend infrastructure and template files for Gallus CMS.

- Deleted `backend/.env.local`, admin page, and OAuth callback page.
- Removed legacy Docker configurations [`docker-compose.yml`, `Dockerfile.caddy`].
- Deprecated migration script, unused routes, and event/gallery migration documentation.
- Updated gitignore to reflect removed folder structure.

README.md

@@ -45,4 +45,3 @@ All commands are run from the root of the project, from a terminal:
 ## 👀 Want to learn more?
 
 Feel free to check [our documentation](https://docs.astro.build) or jump into our [Discord server](https://astro.build/chat).
-# Test commit to trigger Woodpecker