chore: update default catalog to anvil15 (#4902)

[NoopDog]

Closes #4902

What changed

Updated the catalog pinned for the production AnVIL Explorer from anvil14 to anvil15 in site-config/anvil-cmg/prod/config.ts, and extracted the value into a named CATALOG constant to match the pattern used by the hca-dcp and lungmap prod configs.

Why

The Azul backend team released the anvil15 catalog and confirmed on #4902 that it is ready (the mirroring blocker was removed since there is no new public data to mirror). The prod Explorer must point at the new catalog to serve it.

Assumptions I made

• The definition of done (inferred — the issue body is empty) is that the prod AnVIL Explorer queries the anvil15 catalog, matching how the anvil13→anvil14 release ([AnVIL DX] Update default catalog to anvil14 #4883/chore: update default catalog to anvil14 #4887) was done.
• Only the prod config pins a catalog; dev/cc-dev environments intentionally fall back to the backend default and were left unchanged.
• No frontend code changes are needed for anvil15 — per Hannes on the issue, there is no new public data, and no schema/field changes were mentioned.

How to verify

The issue has no definition-of-done checklist; the inferred criterion is: prod AnVIL Explorer serves the anvil15 catalog.

1. Build or deploy this branch with the prod config (npm run build:anvil-cmg), or wait for the prod deploy.
2. Open the Datasets list on the deployed Explorer and confirm data loads without errors.
3. In the browser dev tools Network tab, confirm requests to https://service.explore.anvilproject.org include catalog=anvil15.
4. Spot-check a dataset detail page and a file export flow to confirm the new catalog responds normally.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Updates the production AnVIL CMG site configuration to pin the Explorer’s Azul catalog to the newly released anvil15, aligning with the existing “named CATALOG constant” pattern used in other prod configs.

Changes:

• Bumped the prod AnVIL CMG catalog from anvil14 to anvil15.
• Extracted the catalog string into a CATALOG constant and passed it into makeConfig(...).

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