fix(posthog): read server PostHog config from env vars directly

The PR removed runtimeConfig.public.posthog from nuxt.config.ts since
@posthog/nuxt manages it via posthogConfig. However server/utils/posthog.ts
still referenced config.public.posthog?.publicKey/host which TypeScript
no longer recognises (TS2339).

Switch to process.env.POSTHOG_PUBLIC_KEY / POSTHOG_HOST directly — this is
server-only code running in Nitro, which has full access to process.env,
and avoids the redundant runtimeConfig round-trip.

Author: JoachimLK

server/utils/posthog.ts

@@ -12,9 +12,8 @@ let client: PostHog | null = null
 export function useServerPostHog(): PostHog | null {
   if (client) return client
 
-  const config = useRuntimeConfig()
-  const publicKey = config.public.posthog?.publicKey
-  const host = config.public.posthog?.host
+  const publicKey = process.env.POSTHOG_PUBLIC_KEY
+  const host = process.env.POSTHOG_HOST
 
   if (!publicKey) {
     return null