feat: 为 redis_bucket.go 添加限流日志

添加 trace-aware 日志：
- Error: Redis 限流检查失败（fail-open 降级）
- Warn: 限流触发，记录 key 和 retry_after_sec

backend/internal/ratelimit/redis_bucket.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/hhs/camtalk/internal/config"
+	"github.com/hhs/camtalk/internal/trace"
 	"github.com/redis/go-redis/v9"
 )
 
@@ -72,6 +73,7 @@ func NewRedisLimiter(client *redis.Client, cfg config.RateLimitConfig) *RedisLim
 
 // Allow 实现 Limiter 接口。
 func (l *RedisLimiter) Allow(ctx context.Context, key string) (bool, time.Duration) {
+	log := trace.FromContext(ctx)
 	cfg := l.getBucketConfig(key)
 
 	now := float64(time.Now().UnixNano()) / 1e9 // 秒，浮点
@@ -81,6 +83,7 @@ func (l *RedisLimiter) Allow(ctx context.Context, key string) (bool, time.Durati
 		cfg.Capacity, cfg.Rate, now, ttl).Result()
 
 	if err != nil {
+		log.Errorw("rate limit check failed", "key", key, "error", err)
 		// Redis 错误时降级：允许请求（fail-open 策略）
 		return true, 0
 	}
@@ -100,6 +103,7 @@ func (l *RedisLimiter) Allow(ctx context.Context, key string) (bool, time.Durati
 	}
 
 	retryAfter := time.Duration(retryAfterSec*1000) * time.Millisecond
+	log.Warnw("rate limit triggered", "key", key, "retry_after_sec", retryAfterSec)
 	return false, retryAfter
 }