feat(billing): implement US-3.7 billing and subscription UX with detailed dashboard, real-time invoice history, inline paywall and upgrade confirmation

memento-note/app/api/billing/status/route.ts

@@ -22,6 +22,7 @@ export async function GET() {
       effectiveTier,
       status,
       currentPeriodEnd: currentPeriodEnd ?? null,
+      currentPeriodStart: subscription?.currentPeriodStart ?? null,
       cancelAtPeriodEnd: subscription?.cancelAtPeriodEnd ?? false,
       hasStripeSubscription: !!subscription?.stripeSubscriptionId,
     });