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/Dockerfile

@@ -0,0 +1,17 @@
+FROM python:3.11-slim
+
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    gcc \
+    libpq-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY app ./app
+
+EXPOSE 8001
+
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8001"]