ci(docker): drop -v, -n auto instead of -n 30, pip cache mount

Three things were making the Docker test runs noisier and slower than
  they needed to be:

  1. -v was hardcoded in Dockerfile.test:35 CMD and in docker-compose.
     test.yml's integration-test-runner command. The ci.yml change to
     drop -v from the bare pytest call missed both — Docker runs use
     the image's CMD, not the workflow's.

  2. -n 30 was hardcoded as the xdist worker count. On a 2-vCPU CI box
     that's 30 Python processes fighting over 2 cores — mostly IPC and
     import-thrash overhead. -n auto adapts to the host: 2 on CI, 30
     on a 30-core dev box. Same final-result throughput on the dev
     box, much better on small runners.

  3. pip install had --no-cache-dir and no BuildKit cache mount, so
     every Docker build re-fetched ~50 packages from PyPI (~60-90s
     on a cold pip cache). Adding `RUN --mount=type=cache,target=
     /root/.cache/pip` (with the `# syntax=docker/dockerfile:1.7`
     directive that enables it) makes subsequent builds re-use the
     download cache so they only do install work, ~5s instead of
     ~90s. DOCKER_BUILDKIT=1 is already exported in test_docker.sh
     and is the GHA default since runner image 2023, so the cache
     mount is always honoured.

  Verified locally: Docker build is 19s warm (was ~90s cold each
  time), test run is 102s with 5287 passed / 1 skipped (the
  by-design spoolbuddy importorskip) — clean output, no [gwN]
  worker spam, no "created: 30/30 workers" startup line.

  GHA-side per-run cold-build slowness still happens because GHA
  runners are ephemeral; a follow-up using docker/build-push-action
  with type=gha cache backend would persist the BuildKit cache
  across CI runs but that's a bigger workflow change.

docker-compose.test.yml

@@ -56,7 +56,9 @@ services:
     environment:
       - BAMBUDDY_TEST_URL=http://integration:8000
       - TESTING=1
-    command: ["pytest", "backend/tests/integration/", "-v", "--tb=short", "-p", "no:cacheprovider", "-n", "30"]
+    # -v dropped + -n auto so integration tests inherit the same noise /
+    # parallelism profile as the backend-test image (see Dockerfile.test CMD).
+    command: ["pytest", "backend/tests/integration/", "--tb=short", "-p", "no:cacheprovider", "-n", "auto"]
     volumes:
       - ./backend:/app/backend:ro