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.

fly.toml

@@ -9,9 +9,6 @@ kill_timeout = 5
 [env]
   PORT = "3000"
   NODE_ENV = "production"
-  BACKEND_PORT = "8080"
-  DATABASE_PATH = "/app/data/db/gallus_cms.db"
-  GIT_WORKSPACE_DIR = "/app/data/workspace"
 
 [http_service]
   internal_port = 3000
@@ -42,8 +39,4 @@ kill_timeout = 5
 [[vm]]
   memory = "512MB"
   cpu_kind = "shared"
-  cpus = 1
-
-[[mounts]]
-  source = "gallus_data"
-  destination = "/app/data"
+  cpus = 1