Add health check endpoint and resolve locale detection bug

[dixneuf19]

Hi,

After being impacted by #221 I wanted to contribute to this great replacement for Tricount.
I let loose Cursor to get this very basic implementation for:

• A health check endpoint, useful in environments such as Kubernetes

I differentiated "liveness" and "readiness" probe but for most cases just the /api/health will be enough.

• Fix the locale selection with a sane default in case of error --> default to en-US

New Endpoints

GET /api/health           # Default (same as readiness) - for monitoring tools
GET /api/health/liveness  # App health only - for Kubernetes liveness probe
GET /api/health/readiness # All dependencies - for Kubernetes readiness probe

The response looks like this (code 200 or 503). We could add test to other external services such as S3/openAI but I think it would be overkill for the moment

{
  "status": "healthy",
  "services": {
    "database": {
      "status": "healthy"
    }
  }
}

Local Testing

• curl http://localhost:3000 - No more 500 errors with missing Accept-Language
• All health endpoints work correctly with/without database

Let me know if you want to spliit this into two PR, but it should close #221.