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.

tests/src/smtp/inbound/antispam.rs

@@ -10,9 +10,14 @@ use common::enterprise::llm::{
 };
 #[cfg(feature = "pending-rebuild")]
 use spam_filter::analysis::llm::SpamFilterAnalyzeLlm;
+#[cfg(feature = "pending-rebuild")]
+use crate::utils::http_server::{HttpMessage, spawn_mock_http_server};
+#[cfg(feature = "pending-rebuild")]
+use http_proto::{JsonResponse, ToHttpResponse};
+#[cfg(feature = "pending-rebuild")]
+use hyper::Method;
 use crate::utils::{
     dns::DnsCache,
-    http_server::{HttpMessage, spawn_mock_http_server},
     server::TestServerBuilder,
 };
 use ahash::AHashSet;
@@ -21,8 +26,6 @@ use common::{
     auth::{AccountCache, AccountInfo},
     config::mailstore::spamfilter::SpamFilterAction,
 };
-use http_proto::{JsonResponse, ToHttpResponse};
-use hyper::Method;
 use mail_auth::{
     ArcOutput, DkimOutput, DkimResult, DmarcResult, DnssecStatus, IprevOutput, IprevResult, MX,
     SpfOutput, SpfResult, dkim::Signature, dmarc::Policy,