🔧 Flaky perf test — relax hardcoded <50ms bound on shared runners

[PierreBrisorgueil]

Problem

A performance integration test (billing.extraBalance.listLedger.perf.integration.tests.js) asserts listLedgerPage fetches page 1 of a 1000-entry ledger in < 50ms, a hardcoded wall-clock bound. On shared CI runners (ARC on a busy K8s node) it flakes under load — observed 86ms on a green, correct build, failing an unrelated PR.

Fix (pick one)

• Gate on a percentile / relative bound instead of an absolute 50ms wall-clock.
• Bump the ceiling to a load-tolerant value, or
• Mark it perf-flaky: retry-once, or skip when the runner is under contention.

The query itself isn't slow — the assertion is environment-sensitive. Perf regressions are better caught by a dedicated benchmark job than an inline < 50ms in the integration suite on shared runners.