fix(redis): lock acquisition fails open per spec §9

backend/internal/redispkg/redis.go

@@ -67,8 +67,12 @@ func (r *R) Lock(ctx context.Context, key string, ttl time.Duration) (func(), bo
 	rand.Read(b)
 	tok := hex.EncodeToString(b)
 	ok, err := r.c.SetNX(ctx, key, tok, ttl).Result()
-	if err != nil || !ok {
-		return noop, false
+	if err != nil { // spec §9: Redis 故障降级放行,锁只做尽力去重
+		log.Printf("redis lock %s: %v (proceeding without lock)", key, err)
+		return noop, true
+	}
+	if !ok {
+		return noop, false // 锁被持有,别的副本在扫
 	}
 	return func() {
 		r.c.Eval(ctx,