build(gate)!: coverage leaves the gate and runs on demand

Both lanes printed a coverage total with no threshold (owner decision E32
D4). No real hit as a lane: the one time the number found anything (5 bugs,
E32) it was read by hand, and since then it was a line nobody acted on. It
was also the most expensive part of the vitest lane, and the run that
flaked in E35 S1. The vitest lane runs without --coverage, the pytest lane
runs plain pytest; docs/development.md gives both on-demand commands, and
the register row says why.

backend/docker-compose.test.yml

@@ -35,9 +35,5 @@ services:
     volumes:
       - ./:/app
     working_dir: /app
-    # The suite, then the coverage total (E32 D4: reported, no threshold). pytest's exit code is the
-    # lane's — the report is printed either way and never decides it.
-    command:
-      - sh
-      - -c
-      - 'python -m pytest --cov=app --cov-report= ; rc=$$?; echo ">> backend coverage (app/) $$(python -m coverage report --format=total)% of lines"; exit $$rc'
+    # Coverage is on demand, not in the gate (E35): see docs/development.md.
+    command: ["python", "-m", "pytest"]