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/fly.toml

@@ -3,8 +3,6 @@ app = "gallus-cms-backend"
 primary_region = "ams"
 
 [build]
-  # Ensure Fly uses the Dockerfile in this backend directory
-  dockerfile = "Dockerfile"
 
 [env]
   PORT = "8080"
@@ -12,10 +10,6 @@ primary_region = "ams"
   GITEA_URL = "https://git.bookageek.ch"
   DATABASE_PATH = "/app/data/gallus_cms.db"
   GIT_WORKSPACE_DIR = "/app/data/workspace"
-  # Cross-site frontend and OAuth
-  FRONTEND_URL = "https://gallus-pub.ch"
-  CORS_ORIGIN = "https://gallus-pub.ch"
-  GITEA_REDIRECT_URI = "https://cms.gallus-pub.ch/api/auth/callback"
 
 [http_service]
   internal_port = 8080