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/gunicorn.conf.py

@@ -0,0 +1,15 @@
+"""Gunicorn configuration for the runtime service.
+
+Workers MUST stay at 1: file leases and Jupyter tickets live in
+in-memory process state, so a second worker would split that state.
+"""
+
+import os
+
+bind = os.getenv("GUNICORN_BIND", "0.0.0.0:8000")
+workers = int(os.getenv("GUNICORN_WORKERS", "1"))
+worker_class = "uvicorn.workers.UvicornWorker"
+timeout = int(os.getenv("GUNICORN_TIMEOUT", "120"))
+graceful_timeout = int(os.getenv("GUNICORN_GRACEFUL_TIMEOUT", "30"))
+accesslog = "-"
+errorlog = "-"