fix: bind admin OAuth to helper application

[Rexy40407]

What changed

• Defines ADMIN_CLIENT_ID=1526211106081734666 explicitly in the Rust production environment template.
• Defines the GitHub Pages admin origin explicitly.
• Adds a regression test for both production settings.

Why

The owner console signs in through the Vozen Helper Discord OAuth application. After the repository/runtime migration, ADMIN_CLIENT_ID was absent from the Rust production environment, so the runtime fell back to the main Vozen bot CLIENT_ID. Discord tokens minted for the Helper application were then rejected by the audience check even for the correct owner account.

Impact

Owner login at the Vozen admin panel uses the correct OAuth audience and survives future Rust production environment bootstraps/deployments.

Validation

• Site test suite: 6 files passed, 61 tests passed.
• Live production container was recreated with the explicit admin client and origin.
• https://api.vozen.org/health returns 200 {"status":"ok"}.