Validate NIF against AEAT registry

**Public endpoint** to validate a NIF/CIF against the AEAT registry using VeriFactu. This endpoint is useful for: - Pre-validation before registration (proactive feedback) - Validating client NIFs before creating invoices - Verifying that a NIF exists in the registry **Behavior:** - **VALID**: The NIF exists in the AEAT registry. Returns the fiscal name. - **INVALID**: The NIF has correct format but doesn't exist in the registry. - **PENDING**: VeriFactu API is temporarily unavailable. The NIF has valid syntax but couldn't be verified against the registry. - **ERROR**: Technical error during validation (invalid syntax, timeout, etc.) **Note about PENDING:** If you use this endpoint for pre-validation during registration, a PENDING status indicates that the NIF has valid format and the user can continue. The system will validate the NIF automatically when the service becomes available. **Rate limiting:** - With API Key: 100 requests/hour - With Session Cookie: 1000 requests/hour **Authentication:** This endpoint accepts both API Keys and Session Cookies.

curl -X POST "https://app.beel.es/api/v1/nif/validar" \  -H "Content-Type: application/json" \  -d '{    "nif": "B12345678"  }'

{
  "success": true,
  "data": {
    "valido": true,
    "status": "VALID",
    "nombre_fiscal": "JUAN PEREZ GARCIA",
    "mensaje": "NIF successfully validated in AEAT registry",
    "validated_at": "2025-01-15T10:30:00Z"
  },
  "meta": {
    "timestamp": "2025-01-15T10:30:00Z",
    "request_id": "123e4567-e89b-12d3-a456-426614174000"
  }
}

{
  "success": false,
  "error": {
    "code": "BAD_REQUEST",
    "message": "Invalid request"
  },
  "meta": {
    "timestamp": "2025-01-15T10:30:00Z",
    "request_id": "123e4567-e89b-12d3-a456-426614174000"
  }
}

{
  "success": false,
  "error": {
    "code": "UNAUTHORIZED",
    "message": "Authentication required"
  },
  "meta": {
    "timestamp": "2025-01-15T10:30:00Z",
    "request_id": "123e4567-e89b-12d3-a456-426614174000"
  }
}

{
  "success": false,
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Validation error",
    "details": {
      "field_name": "Field is required"
    }
  },
  "meta": {
    "timestamp": "2025-01-15T10:30:00Z",
    "request_id": "123e4567-e89b-12d3-a456-426614174000"
  }
}

{
  "success": false,
  "error": {
    "code": "INTERNAL_ERROR",
    "message": "Internal server error"
  },
  "meta": {
    "timestamp": "2025-01-15T10:30:00Z",
    "request_id": "123e4567-e89b-12d3-a456-426614174000"
  }
}