feat!: backend-only architecture, remove frontend/nginx (v1.2.0)

- Remove frontend/ and nginx/ from core (each customer owns their frontend)
- Update docker-compose.yml to backend-only (postgres + redis + backend)
- Update docker-compose.prod.yml to backend-only
- Add CLAUDE.md with dev guidelines
- Update README.md with new architecture diagram

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nginx/Dockerfile

@@ -1,20 +0,0 @@
-# Stage 1: Build frontend
-FROM node:20-alpine AS frontend-builder
-
-WORKDIR /app
-COPY frontend/package*.json ./
-RUN npm ci
-COPY frontend/ .
-RUN npm run build
-
-# Stage 2: Nginx with frontend + config
-FROM nginx:1.27-alpine
-
-RUN rm /etc/nginx/conf.d/default.conf
-COPY nginx/nginx.conf /etc/nginx/nginx.conf
-COPY --from=frontend-builder /app/dist /usr/share/nginx/html
-
-RUN apk add --no-cache curl
-
-EXPOSE 80
-CMD ["nginx", "-g", "daemon off;"]