perf(仪表盘): 增强统计缓存与隔离配置

新增仪表盘缓存开关与 TTL 配置，支持 Redis key 前缀隔离，并补充单测与校验。

测试: make test-backend

backend/internal/service/dashboard_service_test.go

@@ -0,0 +1,189 @@
+package service
+
+import (
+	"context"
+	"encoding/json"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/usagestats"
+	"github.com/stretchr/testify/require"
+)
+
+type usageRepoStub struct {
+	UsageLogRepository
+	stats  *usagestats.DashboardStats
+	err    error
+	calls  int32
+	onCall chan struct{}
+}
+
+func (s *usageRepoStub) GetDashboardStats(ctx context.Context) (*usagestats.DashboardStats, error) {
+	atomic.AddInt32(&s.calls, 1)
+	if s.onCall != nil {
+		select {
+		case s.onCall <- struct{}{}:
+		default:
+		}
+	}
+	if s.err != nil {
+		return nil, s.err
+	}
+	return s.stats, nil
+}
+
+type dashboardCacheStub struct {
+	get       func(ctx context.Context) (string, error)
+	set       func(ctx context.Context, data string, ttl time.Duration) error
+	getCalls  int32
+	setCalls  int32
+	lastSetMu sync.Mutex
+	lastSet   string
+}
+
+func (c *dashboardCacheStub) GetDashboardStats(ctx context.Context) (string, error) {
+	atomic.AddInt32(&c.getCalls, 1)
+	if c.get != nil {
+		return c.get(ctx)
+	}
+	return "", ErrDashboardStatsCacheMiss
+}
+
+func (c *dashboardCacheStub) SetDashboardStats(ctx context.Context, data string, ttl time.Duration) error {
+	atomic.AddInt32(&c.setCalls, 1)
+	c.lastSetMu.Lock()
+	c.lastSet = data
+	c.lastSetMu.Unlock()
+	if c.set != nil {
+		return c.set(ctx, data, ttl)
+	}
+	return nil
+}
+
+func (c *dashboardCacheStub) readLastEntry(t *testing.T) dashboardStatsCacheEntry {
+	t.Helper()
+	c.lastSetMu.Lock()
+	data := c.lastSet
+	c.lastSetMu.Unlock()
+
+	var entry dashboardStatsCacheEntry
+	err := json.Unmarshal([]byte(data), &entry)
+	require.NoError(t, err)
+	return entry
+}
+
+func TestDashboardService_CacheHitFresh(t *testing.T) {
+	stats := &usagestats.DashboardStats{
+		TotalUsers: 10,
+	}
+	entry := dashboardStatsCacheEntry{
+		Stats:     stats,
+		UpdatedAt: time.Now().Unix(),
+	}
+	payload, err := json.Marshal(entry)
+	require.NoError(t, err)
+
+	cache := &dashboardCacheStub{
+		get: func(ctx context.Context) (string, error) {
+			return string(payload), nil
+		},
+	}
+	repo := &usageRepoStub{
+		stats: &usagestats.DashboardStats{TotalUsers: 99},
+	}
+	cfg := &config.Config{Dashboard: config.DashboardCacheConfig{Enabled: true}}
+	svc := NewDashboardService(repo, cache, cfg)
+
+	got, err := svc.GetDashboardStats(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, stats, got)
+	require.Equal(t, int32(0), atomic.LoadInt32(&repo.calls))
+	require.Equal(t, int32(1), atomic.LoadInt32(&cache.getCalls))
+	require.Equal(t, int32(0), atomic.LoadInt32(&cache.setCalls))
+}
+
+func TestDashboardService_CacheMiss_StoresCache(t *testing.T) {
+	stats := &usagestats.DashboardStats{
+		TotalUsers: 7,
+	}
+	cache := &dashboardCacheStub{
+		get: func(ctx context.Context) (string, error) {
+			return "", ErrDashboardStatsCacheMiss
+		},
+	}
+	repo := &usageRepoStub{stats: stats}
+	cfg := &config.Config{Dashboard: config.DashboardCacheConfig{Enabled: true}}
+	svc := NewDashboardService(repo, cache, cfg)
+
+	got, err := svc.GetDashboardStats(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, stats, got)
+	require.Equal(t, int32(1), atomic.LoadInt32(&repo.calls))
+	require.Equal(t, int32(1), atomic.LoadInt32(&cache.getCalls))
+	require.Equal(t, int32(1), atomic.LoadInt32(&cache.setCalls))
+	entry := cache.readLastEntry(t)
+	require.Equal(t, stats, entry.Stats)
+	require.WithinDuration(t, time.Now(), time.Unix(entry.UpdatedAt, 0), time.Second)
+}
+
+func TestDashboardService_CacheDisabled_SkipsCache(t *testing.T) {
+	stats := &usagestats.DashboardStats{
+		TotalUsers: 3,
+	}
+	cache := &dashboardCacheStub{
+		get: func(ctx context.Context) (string, error) {
+			return "", nil
+		},
+	}
+	repo := &usageRepoStub{stats: stats}
+	cfg := &config.Config{Dashboard: config.DashboardCacheConfig{Enabled: false}}
+	svc := NewDashboardService(repo, cache, cfg)
+
+	got, err := svc.GetDashboardStats(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, stats, got)
+	require.Equal(t, int32(1), atomic.LoadInt32(&repo.calls))
+	require.Equal(t, int32(0), atomic.LoadInt32(&cache.getCalls))
+	require.Equal(t, int32(0), atomic.LoadInt32(&cache.setCalls))
+}
+
+func TestDashboardService_CacheHitStale_TriggersAsyncRefresh(t *testing.T) {
+	staleStats := &usagestats.DashboardStats{
+		TotalUsers: 11,
+	}
+	entry := dashboardStatsCacheEntry{
+		Stats:     staleStats,
+		UpdatedAt: time.Now().Add(-defaultDashboardStatsFreshTTL * 2).Unix(),
+	}
+	payload, err := json.Marshal(entry)
+	require.NoError(t, err)
+
+	cache := &dashboardCacheStub{
+		get: func(ctx context.Context) (string, error) {
+			return string(payload), nil
+		},
+	}
+	refreshCh := make(chan struct{}, 1)
+	repo := &usageRepoStub{
+		stats:  &usagestats.DashboardStats{TotalUsers: 22},
+		onCall: refreshCh,
+	}
+	cfg := &config.Config{Dashboard: config.DashboardCacheConfig{Enabled: true}}
+	svc := NewDashboardService(repo, cache, cfg)
+
+	got, err := svc.GetDashboardStats(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, staleStats, got)
+
+	select {
+	case <-refreshCh:
+	case <-time.After(1 * time.Second):
+		t.Fatal("等待异步刷新超时")
+	}
+	require.Eventually(t, func() bool {
+		return atomic.LoadInt32(&cache.setCalls) >= 1
+	}, 1*time.Second, 10*time.Millisecond)
+}