Quick fixes from the first boot: recovery admin, upsell, warnings

- The recovery administrator (INBUXA_RECOVERY_ADMIN, or STALWART_RECOVERY_ADMIN)
  is honored only in bootstrap and recovery mode. On a configured server it's
  ignored with a startup warning. Before, it was a standing full-admin login
  for as long as the variable stayed set.
- The Enterprise upsell error is replaced by "This feature isn't available in
  INBUXA yet" for the features still to be rebuilt.
- Workspace warnings: 25 to 0. cargo fix removed the unused imports. The
  seven places where Enterprise code used to plug in keep their parameters,
  each with an inbuxa: comment naming the rebuild that uses it again. The
  antispam test's mock-server imports are back behind pending-rebuild.

crates/common/src/auth/authentication.rs

@@ -514,13 +514,15 @@ impl Server {
 
     pub async fn get_directory_for_domain(
         &self,
-        domain_name: &str,
+        // inbuxa: unused until per-domain directories (Domain.directoryId) are rebuilt; see docs/spec/SPEC.md §4
+        _domain_name: &str,
     ) -> trc::Result<Option<&Arc<Directory>>> {
 
         Ok(self.get_default_directory())
     }
 
-    pub fn get_directory_for_cached_domain(&self, domain: &DomainCache) -> Option<&Arc<Directory>> {
+    // inbuxa: `_domain` is unused until per-domain directories (Domain.directoryId) are rebuilt
+    pub fn get_directory_for_cached_domain(&self, _domain: &DomainCache) -> Option<&Arc<Directory>> {
 
         self.get_default_directory()
     }