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.

Dockerfile.test

@@ -1,4 +1,5 @@
 # Test image for running backend and frontend tests
+# syntax=docker/dockerfile:1.7
 FROM python:3.13-slim AS backend-test
 
 WORKDIR /app
@@ -8,10 +9,15 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     curl \
     && rm -rf /var/lib/apt/lists/*
 
-# Install Python dependencies including test dependencies
+# Install Python dependencies including test dependencies.
+# BuildKit cache mount makes subsequent builds re-use the pip download
+# cache so the second build only does install work — the slow ~60-90s
+# fetch step from the first build becomes ~5s. Requires DOCKER_BUILDKIT=1
+# (already set in test_docker.sh).
 COPY requirements.txt ./
 COPY requirements-dev.txt ./
-RUN pip install --no-cache-dir -r requirements.txt -r requirements-dev.txt
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install -r requirements.txt -r requirements-dev.txt
 
 # Copy backend code
 COPY backend/ ./backend/
@@ -31,8 +37,14 @@ ENV PYTHONUNBUFFERED=1
 ENV DATA_DIR=/app/data
 ENV TESTING=1
 
-# Default command runs pytest (excluding docker integration tests)
-CMD ["pytest", "backend/tests/", "-v", "--tb=short", "-p", "no:cacheprovider", "-n", "30"]
+# Default command runs pytest (excluding docker integration tests).
+# -v dropped: 5300+ "PASSED foo::bar" lines per worker eat noticeable
+# stdout I/O time and clutter test_docker.sh output. --tb=short still
+# gives full failure tracebacks when something breaks.
+# -n auto adapts to the host's vCPU count instead of hard-coding 30 —
+# on a 2-vCPU CI / VM runner, -n 30 spawns 30 Python processes fighting
+# for 2 cores, which is mostly IPC + import-thrash overhead.
+CMD ["pytest", "backend/tests/", "--tb=short", "-p", "no:cacheprovider", "-n", "auto"]
 
 # -------------------------------------------
 # Frontend test stage

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