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/handlers/cluster.go

@@ -0,0 +1,32 @@
+package handlers
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"switchboard-core/store"
+)
+
+// ClusterHandler serves the admin cluster API.
+type ClusterHandler struct {
+	stores store.Stores
+}
+
+func NewClusterHandler(stores store.Stores) *ClusterHandler {
+	return &ClusterHandler{stores: stores}
+}
+
+// ListNodes returns all registered cluster nodes.
+// GET /api/v1/admin/cluster
+func (h *ClusterHandler) ListNodes(c *gin.Context) {
+	nodes, err := h.stores.Cluster.ListNodes(c.Request.Context())
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to list cluster nodes"})
+		return
+	}
+	if nodes == nil {
+		nodes = []store.ClusterNode{}
+	}
+	c.JSON(http.StatusOK, gin.H{"data": nodes})
+}