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/database/migrations/postgres/013_cluster_registry.sql

@@ -0,0 +1,13 @@
+-- 013_cluster_registry.sql — v0.6.0
+-- PG-backed cluster registry for node self-assembly.
+-- UNLOGGED: no WAL overhead, contents lost on PG crash (correct behavior —
+-- all nodes are dead anyway and re-register on restart).
+
+CREATE UNLOGGED TABLE IF NOT EXISTS node_registry (
+    node_id       TEXT PRIMARY KEY,
+    endpoint      TEXT NOT NULL DEFAULT '',
+    seq           SERIAL,
+    registered_at TIMESTAMPTZ DEFAULT now(),
+    heartbeat     TIMESTAMPTZ DEFAULT now(),
+    stats         JSONB DEFAULT '{}'
+);