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/config/config.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 	"strconv"
+	"time"
 
 	"github.com/joho/godotenv"
 )
@@ -85,6 +86,17 @@ type Config struct {
 	OIDCRolesClaim   string // JWT claim for role mapping (default "realm_access.roles")
 	OIDCGroupsClaim  string // JWT claim for group sync (default "groups")
 	OIDCAdminRole    string // IdP role value that maps to admin (default "admin")
+
+	// Cluster registry (v0.6.0)
+	// PG-backed node registry for horizontal scaling. No-op on SQLite.
+	// CLUSTER_NODE_ID: override for deterministic identity (default: hostname-PID).
+	// CLUSTER_HEARTBEAT_INTERVAL: tick frequency (default "10s").
+	// CLUSTER_STALE_THRESHOLD: 3x heartbeat = stale (default "30s").
+	// CLUSTER_ENDPOINT: advertised address for peer mesh (Phase 2, auto-detect).
+	ClusterNodeID            string
+	ClusterHeartbeatInterval time.Duration
+	ClusterStaleThreshold    time.Duration
+	ClusterEndpoint          string
 }
 
 // Load reads configuration from environment variables.
@@ -143,6 +155,12 @@ func Load() *Config {
 		OIDCRolesClaim:   getEnv("OIDC_ROLES_CLAIM", "realm_access.roles"),
 		OIDCGroupsClaim:  getEnv("OIDC_GROUPS_CLAIM", "groups"),
 		OIDCAdminRole:    getEnv("OIDC_ADMIN_ROLE", "admin"),
+
+		// Cluster
+		ClusterNodeID:            getEnv("CLUSTER_NODE_ID", ""),
+		ClusterHeartbeatInterval: getEnvDuration("CLUSTER_HEARTBEAT_INTERVAL", 10*time.Second),
+		ClusterStaleThreshold:    getEnvDuration("CLUSTER_STALE_THRESHOLD", 30*time.Second),
+		ClusterEndpoint:          getEnv("CLUSTER_ENDPOINT", ""),
 	}
 }
 
@@ -204,6 +222,15 @@ func getEnvInt(key string, fallback int) int {
 	return fallback
 }
 
+func getEnvDuration(key string, fallback time.Duration) time.Duration {
+	if v := os.Getenv(key); v != "" {
+		if d, err := time.ParseDuration(v); err == nil {
+			return d
+		}
+	}
+	return fallback
+}
+
 func getEnvBool(key string, fallback bool) bool {
 	if v := os.Getenv(key); v != "" {
 		b, err := strconv.ParseBool(v)