Set up E2E testing

[vklimontovich]

Summary

Create an E2E test suite with a real Next.js app and backends running in Docker.

Components

• Test app: Next.js app in e2e/ with all backends configured
• Docker services: ClickHouse, Postgres, MockServer (for API-based backends)
• Playwright for browser tests

Backends to test

Backend	How
ClickHouse	Docker container
Postgres	Docker container (also covers Neon)
Segment	MockServer, verify HTTP calls
GA	MockServer
Mixpanel	MockServer (when implemented)
Amplitude	MockServer (when implemented)
PostHog	Skip (too complex to self-host)

Test scenarios

1. Page view tracking - verify event in ClickHouse/Postgres
2. Custom events via analytics.sendEvent()
3. Client context enrichment (screen size, referrer)
4. Multiple backends simultaneously
5. User identification via getUser callback