feat(backend): add FastAPI backend — procurement, inventory domain, Casdoor auth, PM arrival endpoints

backend/Dockerfile

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+# Backend image — FastAPI + SQLAlchemy async (asyncpg)
+# The PostgreSQL schema is managed by the postgres service (migration.sql +
+# seed.sql init scripts); this image only runs the API.
+FROM python:3.13-slim
+
+ENV PYTHONDONTWRITEBYTECODE=1 \
+    PYTHONUNBUFFERED=1
+
+WORKDIR /srv/backend
+
+# Dependencies first (layer caching)
+# PIP_INDEX_URL overridable (e.g. a China mirror) — default is official PyPI.
+ARG PIP_INDEX_URL=https://pypi.org/simple
+COPY requirements.txt .
+RUN pip install --no-cache-dir -i ${PIP_INDEX_URL} -r requirements.txt
+
+# Application
+COPY app ./app
+COPY tools ./tools
+
+EXPOSE 8000
+
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]