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.

.woodpecker.yml

@@ -1,11 +1,16 @@
 steps:
-  deploy_frontend:
+  deploy:
     image: node:20
-    secrets: [FLY_API_TOKEN]
+    environment:
+      FLY_API_TOKEN:
+        from_secret: FLY_API_TOKEN
     commands:
       - curl -L https://fly.io/install.sh | sh
       - export PATH="$HOME/.fly/bin:$PATH"
-      - flyctl deploy --config fly.toml --app gallus-pub --remote-only
-    when:
-      branch: main
-      event: push
+      - flyctl deploy --config fly.toml --app gallus-pub
+
+when:
+  branch:
+    - main
+  event:
+    - push