Add firehose-based feed that consumes AT Protocol events via Jetstream, replacing polling for dramatically improved performance: - Add internal/firehose package with: - config.go: Jetstream configuration and Arabica collection list - index.go: BoltDB-backed feed index with multi-key indexes - consumer.go: WebSocket consumer with reconnection and backoff - adapter.go: Bridge between firehose and feed service interfaces - Modify feed service to optionally use firehose index when available - Add -firehose CLI flag to enable the feature - Add graceful shutdown handling for firehose consumer Benefits: - Real-time feed updates (<1s latency vs 5min cache) - Zero API calls per feed request (vs ~10N for N users) - Automatic user discovery from firehose events - Fallback to polling if firehose unavailable Co-Authored-By: Claude <noreply@anthropic.com>