feat: add mock Grist server for integration testing

tests/integration/mock_grist/Dockerfile

@@ -0,0 +1,13 @@
+FROM python:3.14-slim
+
+WORKDIR /app
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY server.py .
+
+ENV PORT=8484
+EXPOSE 8484
+
+CMD ["python", "server.py"]