Implement GA4 service with Measurement Protocol calls FRE-5280

- Real GA4 Measurement Protocol implementation (page_view, purchase,
  waitlist_signup, conversion tracking)
- Setup script with manual and automated (GCP Admin API) paths
- GA4 env vars documented in .env.example

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.env.example

@@ -23,3 +23,7 @@ SENTRY_DSN=""
 SENTRY_ENVIRONMENT="development"
 SENTRY_RELEASE="0.1.0"
 SENTRY_TRACES_SAMPLE_RATE="0.1"
+
+# Google Analytics 4
+GA4_MEASUREMENT_ID=""
+GA4_API_SECRET=""