feat: launch Stripe billing flows with lifecycle hardening and analytics

add Stripe checkout, portal, webhook ingestion, and idempotent event persistence

add billing lifecycle state (grace/sync/timeline/admin visibility) and stronger entitlement handling

add analytics event tracking and admin summary APIs plus account/pricing UI integration

shared/analytics/events.ts

@@ -0,0 +1,30 @@
+export type AnalyticsEventName =
+  | 'pricing_plan_selected'
+  | 'checkout_started'
+  | 'checkout_completed'
+  | 'checkout_canceled'
+  | 'portal_opened'
+  | 'quota_warning_shown'
+  | 'quota_exhausted_blocked'
+  | 'feature_gate_encountered'
+  | 'addon_checkout_started'
+  | 'addon_purchase_completed'
+  | 'plan_changed'
+  | 'payment_failed'
+  | 'subscription_canceled';
+
+export type AnalyticsEventSource = 'web_app' | 'api' | 'stripe_webhook' | 'system';
+
+export interface AnalyticsEventInput {
+  eventName: AnalyticsEventName;
+  eventSource: AnalyticsEventSource;
+  userId?: string | null;
+  workspaceId?: string | null;
+  planCode?: string | null;
+  addonCode?: string | null;
+  resource?: string | null;
+  amount?: number | null;
+  currency?: string | null;
+  metadata?: Record<string, unknown>;
+  occurredAt?: string;
+}