Get Repo Health

Return the cached liveness state of a repo by owner/name.

This is intentionally a DB-only lookup, not a live HEAD against GitHub, so it stays cheap to call from the frontend on every paper page render. The nightly repo_health_sweep job is what flips the underlying state.

Response shape: { "full_name": "owner/name", "alive": bool, "stale": bool, # true when last_verified_at is missing or > 14d "http_status": int|null, "last_verified_at": iso|null }

Returns 404 only when we have never heard of the repo at all (so the frontend can't use this endpoint to enumerate our DB).