fix(OAuth缓存): 修复缓存键冲突、401强制刷新及Redis降级处理

- Gemini 缓存键统一增加 gemini: 前缀，避免与其他平台命名空间冲突
- OAuth 账号 401 错误时设置 expires_at=now 并持久化，强制下次请求刷新 token
- Redis 锁获取失败时降级为无锁刷新，仅在 token 接近过期时执行，并检查 ctx 取消状态

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/service/ratelimit_service.go

@@ -85,13 +85,24 @@ func (s *RateLimitService) HandleUpstreamError(ctx context.Context, account *Acc
 
 	switch statusCode {
 	case 401:
-		// 对所有 OAuth 账号在 401 错误时调用缓存失效
+		// 对所有 OAuth 账号在 401 错误时调用缓存失效并强制下次刷新
 		if account.Type == AccountTypeOAuth {
+			// 1. 失效缓存
 			if s.tokenCacheInvalidator != nil {
 				if err := s.tokenCacheInvalidator.InvalidateToken(ctx, account); err != nil {
 					slog.Warn("oauth_401_invalidate_cache_failed", "account_id", account.ID, "error", err)
 				}
 			}
+			// 2. 设置 expires_at 为当前时间，强制下次请求刷新 token
+			if account.Credentials == nil {
+				account.Credentials = make(map[string]any)
+			}
+			account.Credentials["expires_at"] = time.Now().Format(time.RFC3339)
+			if err := s.accountRepo.Update(ctx, account); err != nil {
+				slog.Warn("oauth_401_force_refresh_update_failed", "account_id", account.ID, "error", err)
+			} else {
+				slog.Info("oauth_401_force_refresh_set", "account_id", account.ID, "platform", account.Platform)
+			}
 		}
 		msg := "Authentication failed (401): invalid or expired credentials"
 		if upstreamMsg != "" {