feat: run FastAPI under gunicorn with uvicorn ASGI workers

Production entrypoint switch:
  - pyproject.toml: add gunicorn>=23.0 dep
  - gunicorn.conf.py: env-var-driven config (bind, workers, threads,
    timeout, graceful_timeout, keepalive, log level, access-log format)
  - Dockerfile: CMD gunicorn main:app (auto-loads gunicorn.conf.py from
    WORKDIR /app)
  - docker-compose.yml: forward GUNICORN_WORKERS / GUNICORN_TIMEOUT /
    GUNICORN_BIND
  - .env.example: document the new tunables

Why gunicorn over standalone uvicorn for production:
  - Process supervision: master restarts crashed workers, restarts on
    memory leaks
  - Graceful shutdown: SIGTERM drains workers in-flight
  - Multi-worker: concurrent requests actually run in parallel
  - Standard ops: k8s readiness probes, log aggregators, etc. all know
    gunicorn

Why uvicorn workers (not sync workers): gunicorn can't natively serve
ASGI; uvicorn.workers.UvicornWorker is the canonical way to run an
ASGI app under gunicorn.

Defaults:
  - 2 workers (small MCP service; raise for high concurrency)
  - 1 thread per worker (no blocking I/O)
  - 120s timeout (yarn logs can be slow; uvicorn's 30s default is too
    tight)

Verified: gunicorn boots, lifespan runs (14 tools logged), MCP
initialize + tools/list + tools/call all work, /openapi.json = 200,
multiple gunicorn worker processes visible in ps.

uv sync picked up gunicorn 26.0.0. Tests still 116/116.

Dockerfile

@@ -55,11 +55,20 @@ RUN uv sync --index-url=https://pypi.tuna.tsinghua.edu.cn/simple/ --frozen --no-
 COPY main.py ./
 COPY spark_executor ./spark_executor
 COPY common ./common
+COPY gunicorn.conf.py ./
 RUN uv sync --index-url=https://pypi.tuna.tsinghua.edu.cn/simple/ --frozen --no-dev
 
-# Put the venv on PATH so `python` / `uvicorn` resolve to the project env.
+# Put the venv on PATH so `python` / `gunicorn` / `uvicorn` resolve to the project env.
 ENV PATH=/app/.venv/bin:$PATH
 ENV PYTHONUNBUFFERED=1
 
+# gunicorn is the prod entrypoint — multiple ASGI workers, graceful
+# shutdown, stdout/stderr logs. Config knobs are env-var driven (see
+# gunicorn.conf.py).
+#
+# Common overrides via -e flags at `docker run`:
+#   -e GUNICORN_WORKERS=4
+#   -e GUNICORN_TIMEOUT=180
+#   -e GUNICORN_BIND=0.0.0.0:9000
 EXPOSE 8000
-CMD ["python", "main.py"]
+CMD ["gunicorn", "main:app"]