Add GrowthBook self-hosted build args and enable SSE streaming
- Add EXPO_PUBLIC_METRICS_API_HOST, EXPO_PUBLIC_GROWTHBOOK_API_HOST,
and EXPO_PUBLIC_GROWTHBOOK_CLIENT_KEY as Docker build args with
sensible defaults matching upstream Bluesky values
- Write these env vars to .env during build so they're baked in
- Enable streaming: true in GrowthBook SDK init for real-time
feature flag updates via SSE