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.yml

@@ -46,18 +46,6 @@ services:
         condition: service_healthy
     command: uvicorn app.main:app --host 0.0.0.0 --port 8000 --reload
 
-  frontend:
-    build: ./frontend
-    container_name: tianpu_frontend
-    ports:
-      - "3000:3000"
-    volumes:
-      - ./frontend:/app
-      - /app/node_modules
-    depends_on:
-      - backend
-    command: npm run dev
-
 volumes:
   pgdata:
   redisdata: