feat(deploy): add production stack for the VPS

Compose file for ikerdemo.cat with nginx as TLS terminator, the three
.NET services, Zitadel and the storage stack. Values come from
deploy/.env.prod (untracked). Adds the frontend image, the migration
script and the Zitadel provisioning for production.

frontend/Dockerfile

@@ -0,0 +1,17 @@
+# Build context: frontend/
+FROM node:22-alpine AS build
+WORKDIR /src
+COPY package.json package-lock.json ./
+RUN npm install
+COPY . .
+RUN npm run build
+
+FROM node:22-alpine AS runtime
+WORKDIR /app
+ENV NODE_ENV=production
+COPY --from=build /src/build ./build
+COPY --from=build /src/node_modules ./node_modules
+COPY --from=build /src/package.json ./package.json
+EXPOSE 3000
+ENV PORT=3000
+CMD ["node", "build/index.js"]