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.

backend/.gitignore

@@ -4,9 +4,7 @@ dist
 *.log
 .DS_Store
 /tmp
-/data/*.db
-/data/*.db-wal
-/data/*.db-shm
-/data/workspace
-# Allow images to be committed
-!/data/images
+/data
+*.db
+*.db-wal
+*.db-shm