fix(lint): 修复 golangci-lint 问题（gofmt + staticcheck）

backend/internal/service/gemini_messages_compat_service.go

@@ -1924,7 +1924,7 @@ func (s *GeminiMessagesCompatService) handleGeminiUpstreamError(ctx context.Cont
 				cooldown = s.rateLimitService.GeminiCooldown(ctx, account)
 			}
 			ra = time.Now().Add(cooldown)
-			log.Printf("[Gemini 429] Account %d (Code Assist, tier=%s, project=%s) rate limited, cooldown=%v", account.ID, tierID, projectID, ra.Sub(time.Now()).Truncate(time.Second))
+			log.Printf("[Gemini 429] Account %d (Code Assist, tier=%s, project=%s) rate limited, cooldown=%v", account.ID, tierID, projectID, time.Until(ra).Truncate(time.Second))
 		} else {
 			// API Key / AI Studio OAuth: PST 午夜
 			if ts := nextGeminiDailyResetUnix(); ts != nil {