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)

app/Models/User.php

@@ -64,6 +64,16 @@ class User extends Authenticatable implements FilamentUser, LdapAuthenticatable
         return $this->hasRole('administrator');
     }
 
+    public function getLdapGuidColumn(): string
+    {
+        return 'ldap_guid';
+    }
+
+    public function getLdapDomainColumn(): string
+    {
+        return 'ldap_domain';
+    }
+
     public function emergencyContacts(): HasMany
     {
         return $this->hasMany(EmergencyContact::class);