feat(fase2): add Flow models, Flow Engine setup, update Docker Compose

- Add Flow and FlowSession SQLAlchemy models for chatbot flows
- Add TriggerType enum (welcome, keyword, fallback, event, manual)
- Setup flow-engine service with FastAPI structure
- Add flow-engine to docker-compose.yml

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -81,6 +81,25 @@ services:
     networks:
       - wac_network
 
+  flow-engine:
+    build:
+      context: ./services/flow-engine
+      dockerfile: Dockerfile
+    container_name: wac_flow_engine
+    restart: unless-stopped
+    environment:
+      DATABASE_URL: postgresql://${DB_USER:-whatsapp_admin}:${DB_PASSWORD}@postgres:5432/${DB_NAME:-whatsapp_central}
+      REDIS_URL: redis://redis:6379
+      API_GATEWAY_URL: http://api-gateway:8000
+      WHATSAPP_CORE_URL: http://whatsapp-core:3001
+    depends_on:
+      postgres:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+    networks:
+      - wac_network
+
   frontend:
     build:
       context: ./frontend