FastAPI application skeleton with health endpoint and settings loading

[Ryan-Atkinson87]

Spec: §2 (FastAPI backend — REST + SSE), §13 (Tech stack).

The deterministic engine's HTTP surface. A FastAPI app with a health check and environment-driven settings, structured so the SSE live-progress stream and REST routes can be added later without rework.

Acceptance criteria

• FastAPI app factory (create_app()), runnable via uvicorn.
• GET /health returns 200 with a small status JSON.
• Settings loaded from the environment (Pydantic settings or equivalent); no hardcoded config values, no hardcoded URLs.
• Router layout in place to host both REST routes and a future SSE stream.
• App config appropriate for being served behind Traefik (§2).
• Unit test asserts /health returns 200.

Dependencies

Depends on: #1