perf: 优化后端性能与直播延迟，包含数据库WAL模式、流媒体写缓冲、转码 Preset 优化、聊天锁和指标采集优化，以及前端自动追帧功能

backend/internal/monitor/metrics.go

@@ -2,6 +2,7 @@ package monitor
 
 import (
 	"runtime"
+	"sync"
 	"sync/atomic"
 	"time"
 )
@@ -23,21 +24,32 @@ type Snapshot struct {
 	ErrorsTotal   uint64  `json:"errors_total"`
 }
 
-func IncrementRequestCount() {
-	atomic.AddUint64(&totalRequests, 1)
+var (
+	cachedSnapshot Snapshot
+	snapshotMutex  sync.RWMutex
+)
+
+func init() {
+	// Initialize the snapshot once on startup
+	updateSnapshot()
+
+	// Update the snapshot in the background every 2 seconds to avoid STW runtime.ReadMemStats in request threads
+	go func() {
+		ticker := time.NewTicker(2 * time.Second)
+		for range ticker.C {
+			updateSnapshot()
+		}
+	}()
 }
 
-func IncrementErrorCount() {
-	atomic.AddUint64(&totalErrors, 1)
-}
-
-func GetSnapshot() Snapshot {
+func updateSnapshot() {
 	var mem runtime.MemStats
 	runtime.ReadMemStats(&mem)
 
 	diskTotal, diskFree := getDiskSpaceGB()
 
-	return Snapshot{
+	snapshotMutex.Lock()
+	cachedSnapshot = Snapshot{
 		UptimeSeconds: int64(time.Since(startedAt).Seconds()),
 		Goroutines:    runtime.NumGoroutine(),
 		MemoryAllocMB: bytesToMB(mem.Alloc),
@@ -48,6 +60,28 @@ func GetSnapshot() Snapshot {
 		RequestsTotal: atomic.LoadUint64(&totalRequests),
 		ErrorsTotal:   atomic.LoadUint64(&totalErrors),
 	}
+	snapshotMutex.Unlock()
+}
+
+func IncrementRequestCount() {
+	atomic.AddUint64(&totalRequests, 1)
+}
+
+func IncrementErrorCount() {
+	atomic.AddUint64(&totalErrors, 1)
+}
+
+func GetSnapshot() Snapshot {
+	snapshotMutex.RLock()
+	defer snapshotMutex.RUnlock()
+
+	// Return the cached snapshot, overlaying volatile/cheap fields in real-time
+	s := cachedSnapshot
+	s.UptimeSeconds = int64(time.Since(startedAt).Seconds())
+	s.Goroutines = runtime.NumGoroutine()
+	s.RequestsTotal = atomic.LoadUint64(&totalRequests)
+	s.ErrorsTotal = atomic.LoadUint64(&totalErrors)
+	return s
 }
 
 func bytesToMB(v uint64) float64 {