[api] Add GET /ready readiness endpoint (PRD PR-4)

[yinkscss]

Context

PRD v0.2 AC-F6.3 and launch gate PR-4 require a readiness endpoint that returns 200 only when the DB pool and required config are initialized.

Problem

Gateway and API expose /health but no dedicated /ready signal for Render and orchestrators to distinguish liveness from readiness.

Proposed scope

• Add GET /ready to crates/gateway and crates/api
• Return 503 when DB unreachable or required env vars missing in production mode
• Document behavior in maintainer runbook ([documentation] Add Render production deployment runbook #39)

Acceptance criteria

• /ready returns 200 when DB ping succeeds and required config present
• /ready returns 503 when DB unreachable
• Render health checks can target /ready separately from /health

References

• docs/PRD.md (AC-F6.3, PR-4)
• crates/api/src/routes/mod.rs
• crates/gateway/src/routes.rs