Feat v0.6.0 cluster registry + HA

PG-backed cluster registry for horizontal scaling — zero new
infrastructure (no etcd/Consul/Redis). MVP convergence point.

- node_registry UNLOGGED table (migration 013)
- Self-registration, heartbeat tick (10s), stale sweep (30s),
  self-eviction (os.Exit on 0 rows → K8s restarts)
- GET /api/v1/admin/cluster returns nodes with runtime stats
- Health endpoints include node_id + cluster size/peers
- cluster-dashboard admin surface with auto-refresh
- 3-replica E2E test (docker-compose-e2e.yml + ci/e2e-cluster-test.sh)
- chat + cluster-dashboard added to curated default bundle
- 5 new tests (3 unit + 2 handler), all passing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/store/postgres/cluster.go

@@ -0,0 +1,78 @@
+package postgres
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"switchboard-core/store"
+)
+
+// ClusterStore manages node_registry — Postgres-only UNLOGGED table.
+type ClusterStore struct{}
+
+func NewClusterStore() *ClusterStore { return &ClusterStore{} }
+
+func (s *ClusterStore) Register(ctx context.Context, nodeID, endpoint string) error {
+	_, err := DB.ExecContext(ctx, `
+		INSERT INTO node_registry (node_id, endpoint)
+		VALUES ($1, $2)
+		ON CONFLICT (node_id) DO UPDATE
+		SET endpoint      = EXCLUDED.endpoint,
+		    registered_at = now(),
+		    heartbeat     = now(),
+		    stats         = '{}'
+	`, nodeID, endpoint)
+	return err
+}
+
+func (s *ClusterStore) Heartbeat(ctx context.Context, nodeID string, stats json.RawMessage) (int64, error) {
+	result, err := DB.ExecContext(ctx, `
+		UPDATE node_registry
+		SET heartbeat = now(), stats = $2
+		WHERE node_id = $1
+	`, nodeID, stats)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected()
+}
+
+func (s *ClusterStore) SweepStale(ctx context.Context, threshold time.Duration) (int64, error) {
+	result, err := DB.ExecContext(ctx, `
+		DELETE FROM node_registry
+		WHERE heartbeat < now() - $1 * interval '1 second'
+	`, threshold.Seconds())
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected()
+}
+
+func (s *ClusterStore) ListNodes(ctx context.Context) ([]store.ClusterNode, error) {
+	rows, err := DB.QueryContext(ctx, `
+		SELECT node_id, endpoint, seq, registered_at, heartbeat, stats
+		FROM node_registry
+		ORDER BY seq
+	`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var nodes []store.ClusterNode
+	for rows.Next() {
+		var n store.ClusterNode
+		if err := rows.Scan(&n.NodeID, &n.Endpoint, &n.Seq, &n.RegisteredAt, &n.Heartbeat, &n.Stats); err != nil {
+			return nil, fmt.Errorf("scan cluster node: %w", err)
+		}
+		nodes = append(nodes, n)
+	}
+	return nodes, rows.Err()
+}
+
+func (s *ClusterStore) Deregister(ctx context.Context, nodeID string) error {
+	_, err := DB.ExecContext(ctx, `DELETE FROM node_registry WHERE node_id = $1`, nodeID)
+	return err
+}