-- Migration 031 — Add last_seen_at to cluster_nodes
--
-- Playout failover (routes/playout.js restartChannel) queries cluster_nodes.last_seen_at
-- to find healthy nodes for channel re-placement. Column was missing from original
-- cluster schema; heartbeat endpoint updates it via /cluster/heartbeat.

ALTER TABLE cluster_nodes ADD COLUMN IF NOT EXISTS last_seen_at TIMESTAMPTZ;

-- Backfill existing nodes to NOW() so they're immediately eligible for failover
UPDATE cluster_nodes SET last_seen_at = NOW() WHERE last_seen_at IS NULL;