demo-ui: deploy an authenticated sandbox showcase with rate limits and observability

[teesofttech]

Why

GitHub Pages can host documentation but not the server-side SDK or secret-backed provider calls. A working showcase needs a separate backend deployment with strict sandbox-only controls.

Scope

• Select a low-cost hosting target for the ASP.NET Core example backend and static UI.
• Enforce sandbox-only provider configuration.
• Add authentication, rate limiting, budgets/quotas, health checks, and redacted telemetry.
• Use ephemeral or regularly cleaned test data.
• Link the showcase from docs with availability expectations.

Acceptance criteria

• No live provider credential is present.
• Anonymous abuse and cost exposure are bounded.
• Health and provider degradation are visible.
• Deployment and rollback are automated.
• The docs site remains useful when the showcase is unavailable.