perf(clickhouse): P0 dashboard/observability query limits, cache, slower polling

internal/apps/openflare/dashboard/cache.go

@@ -0,0 +1,33 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package dashboard
+
+import (
+	"sync"
+	"time"
+)
+
+const overviewCacheTTL = 30 * time.Second
+
+var overviewCache struct {
+	mu        sync.Mutex
+	payload   *OverviewPayload
+	expiresAt time.Time
+}
+
+func getCachedOverview() (*OverviewPayload, bool) {
+	overviewCache.mu.Lock()
+	defer overviewCache.mu.Unlock()
+	if overviewCache.payload == nil || time.Now().After(overviewCache.expiresAt) {
+		return nil, false
+	}
+	return overviewCache.payload, true
+}
+
+func setCachedOverview(payload *OverviewPayload) {
+	overviewCache.mu.Lock()
+	defer overviewCache.mu.Unlock()
+	overviewCache.payload = payload
+	overviewCache.expiresAt = time.Now().Add(overviewCacheTTL)
+}