P8: Add local upstream smoke test and alpha runbook

[wauputr4]

Goal

Create a repeatable local smoke test using a local OpenAI-compatible upstream and document the alpha validation path.

Acceptance Criteria

• Smoke test starts local API, PostgreSQL, Redis, and a mock or local upstream.
• Test covers models, non-streaming chat, streaming chat, usage, credits, and limits.
• Runbook documents setup, expected output, and troubleshooting.
• Alpha readiness checklist is linked from README or docs.