Fix LDAP authentication failures

- Add missing LDAP env vars to .env.example (host, base DN, bind credentials)
- Use 'uid' instead of 'username' as the LDAP lookup attribute in Auth::attempt
- Override getLdapGuidColumn/getLdapDomainColumn in User model to match
  migration column names (ldap_guid/ldap_domain vs default guid/domain)

resources/views/livewire/auth/login.blade.php

@@ -18,7 +18,7 @@ new #[Layout('components.layouts.guest')] class extends Component {
     {
         $this->validate();
 
-        if (Auth::attempt(['username' => $this->username, 'password' => $this->password], $this->rememberMe)) {
+        if (Auth::attempt(['uid' => $this->username, 'password' => $this->password], $this->rememberMe)) {
             session()->regenerate();
             $this->redirectIntended(route('dashboard'), navigate: true);
             return;