Add Aurora cost optimization rules

What

Aurora-specific rules: idle Aurora clusters and Aurora Serverless v2 minimum-ACU floors set higher than needed.

Why

The RDS module is instance-centric (CLDBRN-AWS-RDS-1..10 target DB instances); Aurora's cluster-level and serverless capacity constructs are unexamined. A Serverless v2 cluster with min ACU of 8 instead of 0.5 bills ~$700/month extra per idle cluster.

Details

• Candidate rules: (1) Aurora cluster idle — no connections over the lookback window (cluster-level analogue of CLDBRN-AWS-RDS-2); (2) Serverless v2 ServerlessV2ScalingConfiguration.MinCapacity above a review threshold — IaC + discovery, reading Terraform aws_rds_cluster / CloudFormation AWS::RDS::DBCluster; (3) optionally, provisioned Aurora cluster whose load fits Serverless v2 — review-severity.
• Decide during research whether rules extend the rds module (service-name alignment: IDs continue CLDBRN-AWS-RDS-N) or open an aurora module; the contiguous-ID convention makes this worth settling before implementation.
• Requires a cluster-level dataset (existing datasets are instance-level).

Start by reading the existing RDS module and the CLDBRN-AWS-RDS-2 rule, then inspect how its instance-level datasets are represented. Research cluster data for Terraform aws_rds_cluster and CloudFormation AWS::RDS::DBCluster, and settle whether the rules belong in rds or a new aurora module; done means the cluster-level dataset and selected rules have a defined implementation path.