PHASE3-Admin1: Admin dashboard + analytics (Posthog)

[psypherion]

• User management (list, search, ban)
• Revenue metrics (MRR, ARPU, churn)
• Usage analytics (which features used)
• Posthog SDK integration
• Cohort analysis (free → pro conversion)
• Funnel tracking (signup → first generation)
• Custom dashboards & reports