build: switch services to gunicorn with per-service config

Replace uvicorn with gunicorn + UvicornWorker in backend, runtime,
and schedule Dockerfiles. New gunicorn.conf.py per service exposes
bind/workers/timeout/graceful_timeout as GUNICORN_* env knobs.

Notable details:
- schedule: timeout=0 (disables gunicorn worker heartbeat) so long
  notebook execution isn't killed by gunicorn's silent-worker kill.
- All three configs: drop dead threads=4 setting; UvicornWorker is
  async and ignores threads.
- Pin gunicorn>=26.0.0 in each pyproject; uv.lock regenerated.
- Drop stale 'COPY contracts ./contracts' from runtime and schedule
  Dockerfiles (contracts dir was deleted in an earlier refactor;
  builds would have failed).
- backend Dockerfile: switch to uv sync layout matching runtime/
  schedule; add build deps for native wheels.

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

runtime/Dockerfile

@@ -20,9 +20,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 
 COPY pyproject.toml uv.lock ./
 COPY common ./common
-COPY contracts ./contracts
 COPY runtime ./runtime
 RUN UV_DEFAULT_INDEX="https://pypi.tuna.tsinghua.edu.cn/simple/" uv sync --frozen --no-dev --no-editable --package runtime
 
 EXPOSE 8000
-CMD ["uv", "run", "--frozen", "--package", "runtime", "uvicorn", "runtime.main:app", "--host", "0.0.0.0", "--port", "8000"]
+CMD ["uv", "run", "--frozen", "--package", "runtime", "gunicorn", "--config", "runtime/gunicorn.conf.py", "runtime.main:app"]