Add GET /health/ready DB-readiness endpoint

[pulzzejason]

Problem

There is no narrow database-readiness probe. The existing GET /api/health aggregates DB + Redis + JWKS checks, which is heavier than what a load balancer / orchestrator wants for a simple "is the DB reachable?" liveness/readiness poll.

Proposal

Add an App Router route handler at src/app/api/health/ready/route.ts that:

• Exports an async GET handler (Next.js 15).
• Uses the shared Prisma singleton (import { db } from "@/lib/db").
• Pings the DB with await db.$queryRaw\SELECT 1``.
• Returns 200 { status: "ok" } on success.
• Returns 503 { status: "error", error: <message> } on failure — never throws / 500s.
• Requires no auth/session (pollable by LBs and orchestrators).

Mirrors the checkDatabase() probe and 200/503 pattern already in src/app/api/health/route.ts.

Out of scope

• Redis / JWKS checks (covered by /api/health).
• Auth or rate limiting.

Engine task: 768f6c2c-f880-4cbe-8fbe-bac777b60925