Add GET /health/ready DB-readiness endpoint

[pulzzejason]

Problem

Load balancers and orchestrators need a narrow, auth-free DB-readiness probe. The existing /api/health bundles DB + Redis + JWKS, heavier than a pure readiness gate.

Acceptance criteria

• GET /api/health/ready route handler exists.
• DB reachable → 200 { status: "ok" }.
• DB unreachable → 503 { status: "error", error }; never throws/500.
• No auth required; uses shared Prisma singleton db from @/lib/db.
• Standard copyright + SPDX header; mirrors checkDatabase() conventions.

Out of scope

• Changing existing /api/health route, Redis/JWKS probes, auth middleware, PM Agent config.

Task #383 — slug add-get-health-ready-db-readiness-endpoint