feat(fase2): add Flow API routes and Flow Engine main app

API Gateway:
- Add flows router with CRUD endpoints
- Add activate/deactivate endpoints
- Auto-deactivate other flows for welcome/fallback triggers

Flow Engine:
- Add main.py with FastAPI app
- Add /process endpoint for message handling
- Add SQLAlchemy models (mirrors api-gateway)

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

services/api-gateway/app/routers/__init__.py

@@ -1,4 +1,5 @@
 from app.routers.auth import router as auth_router
 from app.routers.whatsapp import router as whatsapp_router
+from app.routers.flows import router as flows_router
 
-__all__ = ["auth_router", "whatsapp_router"]
+__all__ = ["auth_router", "whatsapp_router", "flows_router"]