Feat v0.6.4 admin health/metrics tab + cluster merge

Admin Health tab under Monitoring with auto-refreshing runtime, DB,
cluster, and extension metrics panels. New GET /api/v1/admin/metrics
endpoint. Fattened heartbeat JSONB with stack, GC CPU%, extension
count, sandbox stats, trigger fires. Sandbox runner and trigger engine
now track cumulative execution counters. Event bus tracks publish/deliver
counts. Health endpoints consolidated via shared builder. Block renderers
(mermaid, katex, csv-table, diff-viewer) no longer require chat.
cluster-dashboard package retired — merged into admin Health tab.

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

server/events/bus.go

@@ -3,6 +3,7 @@ package events
 import (
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -14,6 +15,8 @@ type Bus struct {
 	subs          map[string][]*subscription
 	seq           uint64 // subscription ID counter
 	broadcastHook func(Event) // called after Publish for cross-pod fan-out; nil-safe
+	publishCount  atomic.Int64
+	deliverCount  atomic.Int64
 }
 
 type subscription struct {
@@ -86,6 +89,8 @@ func (b *Bus) Publish(event Event) {
 // the broadcastHook. Used by the Postgres listener to re-publish remote
 // events without causing an infinite re-broadcast loop.
 func (b *Bus) publishLocal(event Event) {
+	b.publishCount.Add(1)
+
 	b.mu.RLock()
 	var matched []Handler
 	for pattern, subs := range b.subs {
@@ -98,6 +103,7 @@ func (b *Bus) publishLocal(event Event) {
 	b.mu.RUnlock()
 
 	for _, h := range matched {
+		b.deliverCount.Add(1)
 		h(event)
 	}
 }
@@ -126,6 +132,12 @@ func (b *Bus) PublishAsync(event Event) {
 	}
 }
 
+// PublishCount returns the cumulative number of events published.
+func (b *Bus) PublishCount() int64 { return b.publishCount.Load() }
+
+// DeliverCount returns the cumulative number of subscriber deliveries.
+func (b *Bus) DeliverCount() int64 { return b.deliverCount.Load() }
+
 // match checks if a concrete label matches a subscription pattern.
 //
 //	"chat.message.abc" matches "chat.message.abc"  (exact)