Add firehose env vars to the Vine schema
FIREHOSE_WEBHOOK_URL_1K and _10K are the per-threshold Discord webhook
targets; FIREHOSE_JETSTREAM_URL is an optional override for the firehose
consumer jetstream endpoint. All optional so existing deployments keep
booting before Phase 2 lands.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>