fix: registration whitelist (ALLOW_REGISTRATION_EMAIL) never works when registration is disabled

**Describe the bug**

When `DISABLE_USER_REGISTRATION=true` and `ALLOW_REGISTRATION_EMAIL` is set to a whitelist (e.g. `ALLOW_REGISTRATION_EMAIL="alice@example.com"`), the registration whitelist never works: every registration attempt is rejected with `The user registration has been disabled by the administrator. Please contact the administrator!`, even for whitelisted emails.

**Root cause**

In `backend/bizpkg/config/base/base.go`, `getBasicConfigurationFromOldConfig()` populates `AllowRegistrationEmail` from the wrong environment variable:

```go
AllowRegistrationEmail: os.Getenv(consts.DisableUserRegistration),
```

It should read `ALLOW_REGISTRATION_EMAIL` (`consts.AllowRegistrationEmail`). Because `DISABLE_USER_REGISTRATION` is set to `"true"` in this scenario, the whitelist becomes the single-element list `["true"]`, which never contains a real email, so `allowRegisterChecker` always returns false.

**To Reproduce**

1. Set in `.env`: `DISABLE_USER_REGISTRATION=true` and `ALLOW_REGISTRATION_EMAIL="neuroai@neurotrace.net"`
2. Restart the server
3. Try to register an account with `neuroai@neurotrace.net`

**Expected behavior**

Only the email(s) listed in `ALLOW_REGISTRATION_EMAIL` can register; other emails are rejected.

**Actual behavior**

All registration attempts (including whitelisted emails) are rejected with "The user registration has been disabled by the administrator."

**Version**

Current `main` (the bug has existed since the persisted base config was introduced, ~PR #2303, Oct 2025).

**Environment**

Any (env-based deployment; independent of the database, since `getBasicConfigurationFromOldConfig()` is the fallback used when no base config is persisted yet).

**Additional context**

- The original feature PR #208 read the correct env var directly; the wrong env var was introduced later when the config moved to the persisted `BasicConfiguration`.
- `AdminAuthMW` already uses `consts.AllowRegistrationEmail` correctly for the admin email fallback; only the registration whitelist path is broken.

Start in backend/bizpkg/config/base/base.go at getBasicConfigurationFromOldConfig() and compare its environment-variable handling with the AdminAuthMW fallback mentioned in the issue. Reproduce with DISABLE_USER_REGISTRATION=true and a whitelisted ALLOW_REGISTRATION_EMAIL; done means the listed email can register while other emails remain rejected.

Written by the indexing model from the issue text.