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>

services/flow-engine/app/__init__.py

@@ -0,0 +1 @@
+# WhatsApp Centralizado - Flow Engine

services/flow-engine/app/config.py

@@ -0,0 +1,17 @@
+from pydantic_settings import BaseSettings
+from functools import lru_cache
+
+
+class Settings(BaseSettings):
+    DATABASE_URL: str = "postgresql://whatsapp_admin:password@localhost:5432/whatsapp_central"
+    REDIS_URL: str = "redis://localhost:6379"
+    API_GATEWAY_URL: str = "http://localhost:8000"
+    WHATSAPP_CORE_URL: str = "http://localhost:3001"
+
+    class Config:
+        env_file = ".env"
+
+
+@lru_cache
+def get_settings() -> Settings:
+    return Settings()