Upload PostHog source maps during builds

[xrendan]

Summary

• add the PostHog Next.js build integration for source-map generation, chunk ID injection, upload, and post-upload deletion
• keep builds working when upload credentials are not configured
• pass the personal API key through a BuildKit secret so it is not persisted in Docker image history or cache

Dokploy setup

The current deployment continues to build if these values are absent; source-map upload is simply skipped. To enable uploads for the Dockerfile build, set:

• Build Time Arguments: POSTHOG_PROJECT_ID=141116
• Build-time Secrets: POSTHOG_API_KEY=

Do not put POSTHOG_API_KEY in Dokploy runtime environment variables or build arguments. Dokploy recommends build-time secrets for sensitive build credentials.

Verification

• pnpm exec eslint next.config.ts
• POSTHOG_API_KEY= POSTHOG_PROJECT_ID= pnpm exec tsc --noEmit
• docker build --check .
• production compilation and type checking pass with upload credentials absent; the complete local build reaches prerendering and then fails because the production API configured for static pages is unavailable locally
• repo-wide lint remains red because ESLint traverses generated files in an existing .claude worktree