Rework accounts-api and auth-api into one service

[katariniss]

Need to rewrite accounts-api and auth-api into one service
So that we have a clean foundation for user management and authentication

Possible Steps

1. Document all endpoints from both services and rename if needed (in this case preserve the old endpoints for backward compatibility)
2. Create new auth-accounts-api repository and setup the project
3. Write down testing strategy and add tests
4. Agree on unified DB schema
5. Implement auth logic (login, register, tokens)
6. Implement account logic (manage accounts, tokens, permissions)
7. Create a plan of the backward compatibility
8. Update auth-ui to use new API endpoints
9. Update accounts-ui to use new API endpoints
10. Remove old services (accounts-api, auth-api)

Definition of Done

1. accounts-api and auth-api are deprecated
2. new unified service (e.g., auth-accounts-api) is ready
3. all original functionality is preserved:

• user registration
• login & logout
• password reset
• email verification
• accounts and tenants management
• roles and permissions management
• JWT token handling

4. new API is backward compatible
5. new service is covered with tests which 100% pass
6. new service follows the patterns of new services that we write (including .Net version 9)

Out of Scope
Possibly some of the steps will be moved to the scope of other tasks. And this task will be decomposed.

[katariniss]

Need to consult Alex on the steps.