chore(dashboard-api): use billing url

[jakubno]

remove the direct dependency on billing server

[cursor]

PR Summary

Medium Risk
Changes Terraform to stop discovering the billing server URL from Cloud Run and instead read it from a Secret Manager secret, which could break deployments if the new secret is missing or misconfigured.

Overview
Updates the GCP Nomad Terraform so dashboard_api no longer depends on looking up the billing service in Cloud Run; it now sources the billing server URL from a Secret Manager secret (while keeping the API token secret-based), reducing direct coupling to the billing server deployment.

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[cursor]

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