feat(billing): promote subscription_changed analytics emit upstream to devkit

[PierreBrisorgueil]

Context

Trawl's pre-/update-stack billing.webhook.service.js emitted subscription_changed PostHog analytics on plan change. This was dropped by /update-stack #1316 (correctly — trawl-specific code). Now promoting to devkit because AnalyticsService is a devkit-level abstraction (Wave 1–4 PostHog integration, Node #3640/#3653).

What

• Add AnalyticsService.isConfigured() to lib/services/analytics.js — returns client !== null; cheap pre-check for callers that want to skip payload construction when PostHog is not active.
• In billing.webhook.service.js handleSubscriptionUpdated: emit subscription_changed analytics event after plan.changed fires, carrying previousPlan/newPlan/isDowngrade; guarded by isConfigured() so downstreams without PostHog are clean no-ops. Non-fatal try/catch.

Why generic + safe

AnalyticsService.isConfigured() returns false when the downstream isn't running PostHog → the capture is a clean no-op. Downstreams using PostHog (trawl, future) get the events automatically after /update-stack.

Cross-ref

• Regression introduced by: /update-stack trawl#1316
• Trawl follow-up: comes-io/trawl_node#1315
• Alignment plan: infra 2026-06-01-trawl-promote-up-followups.md Task 5