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>

schedule/pyproject.toml

@@ -12,11 +12,16 @@ dependencies = [
     "nbclient==0.10.2",
     "nbformat==5.10.4",
     "ipykernel==6.29.5",
+    "gunicorn>=26.0.0",
 ]
 
 [tool.uv.sources]
 common = { workspace = true }
 
+[[tool.uv.index]]
+url = "https://pypi.tuna.tsinghua.edu.cn/simple/"
+default = true
+
 
 [build-system]
 requires = ["hatchling"]