Surface dataset metrics on organization pages

### User story

As an agency data coordinator, I want to see dataset-level click-through metrics on my organization's catalog.data.gov page so that I have a baseline for M-25-05 usage reporting without needing to build my own analytics infrastructure.

### Background

The organization page (e.g., catalog.data.gov/organization/[agency]) is becoming the central hub for agency-level information on Data.gov, including planned additions for SHARE IT Act code repository references. Adding usage metrics continues that pattern and gives agencies a low-barrier starting point for compliance reporting.

Metrics reflect click-throughs recorded in GA4 and will include a disclaimer on scope and limitations.

### Acceptance criteria

- Given a user visits an organization page, when the page loads, then dataset-level click-through metrics are displayed for that organization's datasets.
- Given metrics are displayed, when a user reads the page, then a disclaimer is visible stating that metrics reflect access through catalog.data.gov only and do not reflect direct agency-hosted access.
- Given metrics are displayed, when a user views the data, then metrics are shown across predefined date ranges (specific ranges are a developer decision based on retained data availability).
- Given a user wants to use the data externally, when they click the download button, then a CSV of that organization's dataset metrics downloads, scoped to the selected or default date range.

### Sketch

- Metrics surface on the organization page consistent with the layout pattern being established for SHARE IT code repository references.
- Disclaimer text (draft, subject to edit): "These metrics only reflect data access through the Data.gov platform and do not reflect direct agency-hosted access. For instance, these metrics only show when an item was accessed from catalog.data.gov, not when the item was accessed by someone going to the agency webpage and finding the link."
- CSV download button on organization page.

### Dependencies

- Child ticket: Build GA4 metrics retention system must be complete or in progress before full implementation.
- DCAT-US 3.0 migration should be substantially complete so dataset identifiers are stable.

Start with the organization page and review the “Build GA4 metrics retention system” child ticket, since the retention system must be complete or in progress. Confirm how the DCAT-US 3.0 migration affects dataset identifiers and retained date ranges. Done means organization pages show scoped dataset metrics and a disclaimer, with a CSV download for the selected or default range.

Written by the indexing model from the issue text.