Add option to disable business registry check in the portal

[vohmar]

Background

The registrant portal integrates with the Estonian business registry (äriregister) to
show users the companies they represent and the associated domains. When the business
registry experiences known outages or data quality issues, this integration currently
has no way to be disabled without a code deployment.

Problem

There is no configuration option to disable the business registry check (Companies page
/ /api/companies endpoint) when administrators are aware of problems with that data
source. This forces users to experience degraded or broken portal behaviour during known
business registry incidents.

Proposed Solution

Add an environment variable (e.g. DISABLE_BUSINESS_REGISTRY=true) to the portal
configuration that:

1. Disables the /api/companies endpoint (returns an empty list or a service-unavailable
   response) without affecting any other portal functionality.
2. Optionally renders a configurable maintenance/notice message on the Companies page
   (/companies) so users understand why company data is temporarily unavailable.
3. Is documented in .env.example with a comment explaining when and how to use it.

Acceptance Criteria

• New environment variable DISABLE_BUSINESS_REGISTRY (default: false) controls
  whether the business registry integration is active.
• When disabled, the Companies page shows a user-friendly notice instead of an error
  or empty state.
• Domain listing and all other portal features remain fully functional when the
  business registry check is disabled.
• The variable is documented in .env.example.

Relevant Code

• server/routes/apiRoute.js → getCompanies
• src/pages/CompaniesPage/CompaniesPage.jsx
• src/redux/reducers/companies.js
• .env.example