SaaS analytics dashboard

Build an internal analytics dashboard for a SaaS.

Core features:
1. Cards: MRR, active users, signups today, churn
2. Charts: signups over time, revenue, cohort retention
3. Filter by plan/segment
4. Alerts when a metric drops X% vs 7-day avg
5. Read-only share links

Data: events, users, subscriptions (from Stripe webhook).
Must-haves: fast queries via materialized views, RLS on workspace.