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/repository/dashboard_cache.go

@@ -0,0 +1,51 @@
+package repository
+
+import (
+	"context"
+	"strings"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/redis/go-redis/v9"
+)
+
+const dashboardStatsCacheKey = "dashboard:stats:v1"
+
+type dashboardCache struct {
+	rdb       *redis.Client
+	keyPrefix string
+}
+
+func NewDashboardCache(rdb *redis.Client, cfg *config.Config) service.DashboardStatsCache {
+	prefix := "sub2api:"
+	if cfg != nil {
+		prefix = strings.TrimSpace(cfg.Dashboard.KeyPrefix)
+	}
+	return &dashboardCache{
+		rdb:       rdb,
+		keyPrefix: prefix,
+	}
+}
+
+func (c *dashboardCache) GetDashboardStats(ctx context.Context) (string, error) {
+	val, err := c.rdb.Get(ctx, c.buildKey()).Result()
+	if err != nil {
+		if err == redis.Nil {
+			return "", service.ErrDashboardStatsCacheMiss
+		}
+		return "", err
+	}
+	return val, nil
+}
+
+func (c *dashboardCache) SetDashboardStats(ctx context.Context, data string, ttl time.Duration) error {
+	return c.rdb.Set(ctx, c.buildKey(), data, ttl).Err()
+}
+
+func (c *dashboardCache) buildKey() string {
+	if c.keyPrefix == "" {
+		return dashboardStatsCacheKey
+	}
+	return c.keyPrefix + dashboardStatsCacheKey
+}

backend/internal/repository/wire.go

@@ -58,6 +58,7 @@ var ProviderSet = wire.NewSet(
 	NewAPIKeyCache,
 	NewTempUnschedCache,
 	ProvideConcurrencyCache,
+	NewDashboardCache,
 	NewEmailCache,
 	NewIdentityCache,
 	NewRedeemCache,