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>

docker-compose.prod.yml

@@ -1,28 +1,6 @@
 version: '3.8'
 
 services:
-  nginx:
-    build:
-      context: .
-      dockerfile: nginx/Dockerfile
-    container_name: tianpu_nginx
-    ports:
-      - "80:80"
-    depends_on:
-      backend:
-        condition: service_healthy
-    restart: always
-    healthcheck:
-      test: ["CMD", "curl", "-sf", "http://localhost/health"]
-      interval: 30s
-      timeout: 10s
-      retries: 3
-    logging:
-      driver: json-file
-      options:
-        max-size: "10m"
-        max-file: "3"
-
   postgres:
     image: timescale/timescaledb:latest-pg16
     container_name: tianpu_db