[FEATURE] Add Usage Analytics Dashboard

[JoshuaAFerguson]

Create analytics dashboard for admins.

Metrics:

• Active users (24h, 7d, 30d)
• Sessions created per day
• Average session duration
• Popular templates
• Peak usage hours
• Error rate
• Cost attribution

Acceptance Criteria:

• Dashboard at /admin/analytics
• Real-time metrics
• Historical trends
• Export to CSV

Files: ui/src/pages/Admin/Analytics.tsx (NEW), api/internal/analytics/ (NEW)