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)

.env.example

@@ -63,3 +63,10 @@ AWS_BUCKET=
 AWS_USE_PATH_STYLE_ENDPOINT=false
 
 VITE_APP_NAME="${APP_NAME}"
+
+LDAP_HOST=openldap
+LDAP_USERNAME="cn=admin,dc=travel,dc=local"
+LDAP_PASSWORD=adminpassword
+LDAP_PORT=389
+LDAP_BASE_DN="dc=travel,dc=local"
+LDAP_LOGGING=true