[Sprint 0][P0] Backend key service skeleton

[knzeng-e]

Goal

Create a minimal backend service that becomes the production boundary for upload orchestration, content-key delivery, health checks, and future WebAuthn support.

Instructions

See docs/backlog/01-backend-key-service-skeleton.md.

Acceptance focus

• Backend starts locally.
• /health and /version exist.
• Env config is typed and fail-fast.
• No production secret is added to frontend code.

Engineering bar

Implement as a senior production engineer: typed inputs, explicit errors, safe config, small modules, no magical fallback.