feat(analytics): proxy PostHog ingestion through public-api on Vercel

- Add Vercel rewrites to proxy `/ingest/*` and `/ingest/static/*` to PostHog's US endpoints
- Update native `.env.example` to point `EXPO_PUBLIC_POSTHOG_HOST` at the self-hosted proxy URL

apps/public-api/vercel.json

@@ -1,3 +1,13 @@
 {
-  "bunVersion": "1.x"
+  "bunVersion": "1.x",
+  "rewrites": [
+    {
+      "source": "/ingest/static/:path(.*)",
+      "destination": "https://us-assets.i.posthog.com/static/:path"
+    },
+    {
+      "source": "/ingest/:path(.*)",
+      "destination": "https://us.i.posthog.com/:path"
+    }
+  ]
 }