docs: add comprehensive Stripe billing guide

Covers architecture, configuration steps, user flows, API routes,
webhooks, pricing, testing with Stripe CLI, production checklist,
and troubleshooting.

memento-note/lib/analytics/track.ts

@@ -0,0 +1,17 @@
+/**
+ * Analytics choke-point — all product analytics must go through these helpers.
+ * New third-party scripts in app/ must check hasAnalyticsConsent() before loading.
+ */
+import { hasAnalyticsConsent } from '@/lib/consent/cookie-consent'
+
+export type AnalyticsEventProperties = Record<string, string | number | boolean | null | undefined>
+
+export function trackClientEvent(_event: string, _properties?: AnalyticsEventProperties): void {
+  if (!hasAnalyticsConsent()) return
+  // Future: PostHog/Umami client capture (see saas-deployment-prep.md §D)
+}
+
+export function trackServerEvent(_event: string, _properties?: AnalyticsEventProperties): void {
+  if (!hasAnalyticsConsent()) return
+  // Future: server-side analytics capture
+}