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/cluster_iface.go

@@ -0,0 +1,37 @@
+package store
+
+import (
+	"context"
+	"encoding/json"
+	"time"
+)
+
+// ClusterNode represents a registered node in the cluster registry.
+type ClusterNode struct {
+	NodeID       string          `json:"node_id"`
+	Endpoint     string          `json:"endpoint"`
+	Seq          int             `json:"seq"`
+	RegisteredAt time.Time       `json:"registered_at"`
+	Heartbeat    time.Time       `json:"heartbeat"`
+	Stats        json.RawMessage `json:"stats"`
+}
+
+// ClusterStore manages the node_registry table for cluster self-assembly.
+// Postgres-only — SQLite deployments set this to nil.
+type ClusterStore interface {
+	// Register inserts or re-registers a node (idempotent).
+	Register(ctx context.Context, nodeID, endpoint string) error
+
+	// Heartbeat updates the node's heartbeat timestamp and stats.
+	// Returns rows affected — 0 means the node was swept (self-eviction).
+	Heartbeat(ctx context.Context, nodeID string, stats json.RawMessage) (int64, error)
+
+	// SweepStale deletes nodes whose heartbeat is older than threshold.
+	SweepStale(ctx context.Context, threshold time.Duration) (int64, error)
+
+	// ListNodes returns all registered nodes ordered by sequence.
+	ListNodes(ctx context.Context) ([]ClusterNode, error)
+
+	// Deregister removes a node (best-effort cleanup on shutdown).
+	Deregister(ctx context.Context, nodeID string) error
+}