fix: address code review issues for RPM limiting feature

- Use TxPipeline (MULTI/EXEC) instead of Pipeline for atomic INCR+EXPIRE
- Filter negative values in GetBaseRPM(), update test expectation
- Add RPM batch query (GetRPMBatch) to account List API
- Add warn logs for RPM increment failures in gateway handler
- Reset enableRpmLimit on BulkEditAccountModal close
- Use union type 'tiered' | 'sticky_exempt' for rpmStrategy refs
- Add design decision comments for rdb.Time() RTT trade-off

backend/internal/service/gateway_service.go

@@ -2708,6 +2708,9 @@ func (s *GatewayService) selectAccountForModelWithPlatform(ctx context.Context,
 		}
 	}
 
+	// 批量预取 RPM 计数，避免逐个账号查询（N+1）
+	ctx = s.withRPMPrefetch(ctx, accounts)
+
 	// 3. 按优先级+最久未用选择（考虑模型支持）
 	var selected *Account
 	for i := range accounts {
@@ -2922,6 +2925,9 @@ func (s *GatewayService) selectAccountWithMixedScheduling(ctx context.Context, g
 		}
 	}
 
+	// 批量预取 RPM 计数，避免逐个账号查询（N+1）
+	ctx = s.withRPMPrefetch(ctx, accounts)
+
 	// 3. 按优先级+最久未用选择（考虑模型支持和混合调度）
 	var selected *Account
 	for i := range accounts {