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.

Dockerfile

@@ -2,8 +2,7 @@ FROM node:20-alpine AS build
 
 WORKDIR /app
 COPY package*.json ./
-# Fallback to npm install if no lockfile is present
-RUN npm ci || npm install
+RUN npm ci
 COPY . .
 # Ensure CSS variables are present
 RUN mkdir -p public/styles
@@ -17,8 +16,7 @@ RUN npm install -g serve
 COPY --from=build /app/dist ./dist
 
 EXPOSE 3000
-# Serve static files (no SPA fallback), so /admin serves dist/admin/index.html
-CMD ["serve", "-l", "3000", "dist"]
+CMD ["serve", "-s", "dist", "-l", "3000"]
 
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
   CMD wget -qO- http://localhost:3000/ || exit 1