feat: 升级 cached_user.go 日志为 trace-aware

替换 logger.Log 为 trace.FromContext(ctx)：
- SaveRefreshToken: Redis 缓存写入失败降级日志
- FindRefreshToken: Redis 缓存读取失败降级日志
- DeleteRefreshToken: Redis 缓存删除失败降级日志
- DeleteUserRefreshTokens: Redis 批量删除失败降级日志

backend/internal/store/cached_user.go

@@ -6,7 +6,7 @@ import (
 
 	"github.com/redis/go-redis/v9"
 
-	"github.com/hhs/camtalk/internal/logger"
+	"github.com/hhs/camtalk/internal/trace"
 )
 
 // Redis key 前缀。
@@ -83,7 +83,8 @@ func (r *CachedUserRepository) SaveRefreshToken(ctx context.Context, userID, tok
 	pipe.Set(ctx, key, userID, ttl)
 	pipe.SAdd(ctx, userRefreshKey(userID), tokenHash)
 	if _, err := pipe.Exec(ctx); err != nil {
-		logger.Log.Warnw("Redis cache write failed for refresh token", "error", err)
+		log := trace.FromContext(ctx)
+		log.Warnw("redis cache write failed for refresh token", "error", err)
 		// 降级：DB 已写入成功，Redis 失败不影响正确性
 	}
 	return nil
@@ -100,7 +101,8 @@ func (r *CachedUserRepository) FindRefreshToken(ctx context.Context, tokenHash s
 	}
 	// redis.Nil 表示 key 不存在，其他错误记录日志后降级到 DB
 	if err != redis.Nil {
-		logger.Log.Warnw("Redis cache read failed for refresh token", "error", err)
+		log := trace.FromContext(ctx)
+		log.Warnw("redis cache read failed for refresh token", "error", err)
 	}
 
 	// 降级到 DB
@@ -139,7 +141,8 @@ func (r *CachedUserRepository) DeleteRefreshToken(ctx context.Context, tokenHash
 		pipe.SRem(ctx, userRefreshKey(userID), tokenHash)
 	}
 	if _, err := pipe.Exec(ctx); err != nil {
-		logger.Log.Warnw("Redis cache delete failed for refresh token", "error", err)
+		log := trace.FromContext(ctx)
+		log.Warnw("redis cache delete failed for refresh token", "error", err)
 	}
 	return nil
 }
@@ -159,7 +162,8 @@ func (r *CachedUserRepository) DeleteUserRefreshTokens(ctx context.Context, user
 		}
 		keys = append(keys, userKey)
 		if err := r.rdb.Del(ctx, keys...).Err(); err != nil {
-			logger.Log.Warnw("Redis cache batch delete failed for user refresh tokens", "error", err, "userID", userID)
+			log := trace.FromContext(ctx)
+			log.Warnw("redis cache batch delete failed for user refresh tokens", "error", err, "user_id", userID)
 		}
 	}