feat: add API versioning and error contract

[vinimachadosantana]

What

Adds an /api/v1 namespace, a versioned base controller with centralised error handling, and a JSON spec helper.

Why

Every error response now has one predictable shape, so clients handle failures generically instead of special-casing each endpoint. Versioning means a future breaking change can ship as /api/v2 without stranding existing consumers.

How

• rescue_from handlers are matched bottom-up, so StandardError is declared first and specific exceptions after (500 inverse).
• Unhandled exceptions are logged in full and re-raised in development and test (for a usable stack trace) but return a generic message in production.
• URL versioning over header versioning to be easier to debug and cache.

Testing

• bundle exec rspec green
• RuboCop and Brakeman clean