[M1] Add V2 health and database readiness endpoints

[rodrigolinhas]

Goal

Provide the first infrastructure-level API endpoint proving NestJS and PostgreSQL are wired correctly.

Suggested Distinction

• health (/api/v2/health or /health): application process is alive.
• readiness (/api/v2/readiness or /readiness): required dependencies, including PostgreSQL, are usable.

Do not overengineer observability.

Acceptance Criteria

• Application health endpoint exists.
• Database readiness can be checked.
• Database failure does not produce misleading "healthy" status.
• Appropriate HTTP status codes are used (200 OK vs 503 Service Unavailable).
• Endpoints are documented.
• Automated API tests exist.

Dependencies

• Issue A ([M1] Bootstrap NestJS V2 backend #6): [M1] Bootstrap NestJS V2 backend
• Issue B ([M1] Add PostgreSQL development environment #7): [M1] Add PostgreSQL development environment
• Issue C ([M1] Configure PostgreSQL persistence and versioned migrations #8): [M1] Configure PostgreSQL persistence and versioned migrations
• Issue D ([M1] Add validated configuration for the V2 backend #9): [M1] Add validated configuration for the V2 backend

Suggested PR

PR 3 — Health/readiness + OpenAPI + testing foundation (with Issues F and G)