follow-up: 4 Copilot nits on billing.webhook subscription_changed analytics

[PierreBrisorgueil]

PR #3769 merged with 4 unresolved Copilot nits (acknowledged non-blocking; merged for forward progress):

1. Redundant try/catch around AnalyticsService.capture() — already swallows internally
2. Comment says billing.plan.changed but code emits plan.changed (mismatch breaks grep)
3. Stale-event test fixture doesn't actually exercise the stale-event branch
4. Same-plan test only covers absent previous_attributes.items, not the explicit guard

Cleanup PR to address these. Low priority. Part of plan infra/docs/superpowers/plans/2026-06-01-trawl-promote-up-followups.md Task 5 follow-on.

[PierreBrisorgueil]

Closed by #3773.