Build GA4 metrics retention system

### User Story:
As the Data.gov platform team, we need to retain monthly GA4 click-through metrics at the dataset level so that historical data is available for year-over-year reporting and agency compliance use.

### Background:
GA4 currently provides dataset-level click-through data (instances where a user searched catalog.data.gov and clicked through to access a dataset) but this data is not retained by Data.gov. Without a retention mechanism, historical comparisons are impossible and agencies cannot use Data.gov metrics for M-25-05 compliance reporting. A storage and export process needs to be built so monthly snapshots are captured and queryable.

### Acceptance Criteria:

- Given that a calendar month has ended, when the automated retention job runs, then GA4 dataset-level click-through metrics are exported and stored in a durable, queryable format.
- Given stored metrics, when a developer queries the retention system, then they can retrieve metrics for any dataset by identifier for any retained month.
- Given the retention system is live, when metrics are stored, then data is retained for a minimum of [TBD -- developer recommendation] rolling months to support YOY comparisons.

### Security Considerations:

- Metrics are aggregated and do not contain PII.
- Storage architecture should follow GSA cloud.gov data storage standards.

### Open Questions:

- What is the appropriate retention window?
- What storage mechanism best fits the existing cloud.gov infrastructure?