fix(posthog): replace catch-all with single function + rewrite

test: add /api/ping to verify Vercel function routing

fix(posthog): buffer body before forwarding to fix 405 on POST

Disable Vercel body parser in PostHog proxy function

Fix PostHog proxy: use Node.js stream piping instead of fetch

Switch PostHog proxy from Edge to Node.js serverless function

Replace broken Vercel rewrite proxy with Edge Function for PostHog