Verification feature for api users

[maricavor]

Close internetee/registrar_center2#218

In a Nutshell

This PR implements API user identity verification using eeID (Estonian digital identity service). It adds verification workflows for API users with automatic or manual approval paths, introduces new API endpoints for verification management, and migrates from identity_code to subject as the primary identity identifier while adding comprehensive i18n support throughout the codebase.

Key Changes

New Features:

• API User Verification Endpoints (4 new actions):

  • POST /repp/v1/api_users/verify/:id - Initiate identity verification
  • GET /repp/v1/api_users/download_poi/:id - Download proof of identity PDF
  • POST /repp/v1/api_users/approve_verification/:id - Manual approval with subject override
  • POST /repp/v1/api_users/reject_verification/:id - Reject pending verification

• Webhook Processing - eID webhook handler now supports both Contacts and API users with auto-verification and manual review states

• Email Notifications - New mailer for verification requests and results

Refactoring:

• Replaced identity_code with subject as primary identity field (with backward compatibility via SubjectBackfill utility)
• Enhanced authentication to use eligible_for_sign_in? (requires both active status + verified identity)
• i18n: Externalized ~20+ hardcoded error messages to locale files
• Improved authorization checks and validation logic