Add Mixpanel backend

[vklimontovich]

Summary

Add Mixpanel as a supported analytics backend.

Implementation

Use direct HTTP API via fetch (like segment and posthog backends do).

Why not use a library:

• mixpanel-browser - client-side only, requires DOM
• mixpanel (Node.js SDK) - uses native http/https modules. Won't work on Vercel Edge Runtime.

API endpoint: https://api.mixpanel.com/track (or /import for historical data)

Config

mixpanelBackend({
  token: process.env.MIXPANEL_TOKEN!,
  // optional: use EU data residency
  host?: "https://api-eu.mixpanel.com"
})

Event mapping

• pageView -> Page Viewed event
• custom events -> pass through as-is
• anonymousUserId -> $device_id
• userContext.userId -> $user_id