Enable Playwright's JSON reporter alongside `list` and publish a
summary of flaky tests to the job step summary. The JSON report is
kept as an artifact for 30 days so flakiness rates can be aggregated
over time.
CI already runs with `retries: 2`, so unstable tests have been passing
silently on retry. This only surfaces what the suite already absorbs;
no test behaviour changes.
The reporter in `frontend/scripts/test-e2e` becomes overridable via
`PLAYWRIGHT_REPORTER` so the local developer default stays untouched.
