test: add backend and renderer smoke coverage

[ubugeeei]

Finding

The app can build while the backend/render contract is broken because there is no automated smoke test that starts the backend and verifies health/render endpoints.

Evidence

• backend exposes /healthz, websocket, media, progress, audio plan, and cancel endpoints.
• render/src/main.rs depends on backend URLs and browser automation but has no automated smoke coverage.

Acceptance criteria

• Add a lightweight backend health smoke test.
• Add a renderer contract test or scripted smoke path that avoids full video encoding where possible.
• Document required local binaries/environment variables.

[ubugeeei]

対応済みです。修正は PR #1 / commit 9cf4498 に反映済みです。