refactor: große API-Handler-Dateien in fokussierte Module aufteilen

Reine Code-Verschiebung, keine Logikänderung. Betroffen:
- ldap_tenants.go (994 Zeilen) -> ldap_tenants.go (Routing) + ldap_handlers.go
  + tenant_handlers.go + tenant_domain_handlers.go + tenant_logo_handlers.go
  + tenant_helpers.go
- import_handlers.go (621 Zeilen) -> imap_handlers.go + pop3_handlers.go
  + import_helpers.go
- admin_handlers.go (702 Zeilen) -> admin_users_handlers.go +
  admin_status_handlers.go + admin_services_handlers.go +
  admin_security_handlers.go

Lokal kein Go-Build möglich — Verifikation manuell per Funktions- und
Import-Abgleich Alt/Neu (alle Symbole exakt einmal vorhanden). Build
muss vor Deploy auf 192.168.1.131/132 bestätigt werden.

internal/api/admin_status_handlers.go

@@ -0,0 +1,58 @@
+package api
+
+import (
+	"net/http"
+
+	"archivmail/internal/auth"
+	"archivmail/internal/userstore"
+)
+
+func (s *Server) handleSMTPStatus(w http.ResponseWriter, r *http.Request) {
+	sess := sessionFromCtx(r.Context())
+	tenantID := tenantFromCtx(r.Context())
+
+	// domain_admin: return only their tenant's email statistics (no global daemon info)
+	if sess != nil && !auth.HasRole(sess.Role, userstore.RoleSuperAdmin) {
+		stats, err := s.store.StatsByTenant(r.Context(), tenantID)
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to read stats")
+			return
+		}
+		domains := []string{}
+		if tenantID != nil && s.tenantStore != nil {
+			if dd, derr := s.tenantStore.ListDomains(r.Context(), *tenantID); derr == nil {
+				for _, d := range dd {
+					domains = append(domains, d.Domain)
+				}
+			}
+		}
+		writeJSON(w, http.StatusOK, map[string]interface{}{
+			"enabled":     true,
+			"tenant_only": true,
+			"domains":     domains,
+			"total_mails": stats["count"],
+			"total_bytes": stats["total_size"],
+		})
+		return
+	}
+
+	// superadmin: global daemon status
+	if s.smtpDaemon == nil {
+		writeJSON(w, http.StatusOK, map[string]interface{}{"enabled": false, "running": false})
+		return
+	}
+	writeJSON(w, http.StatusOK, s.smtpDaemon.Status())
+}
+
+func (s *Server) handleStorageStats(w http.ResponseWriter, r *http.Request) {
+	tenantID := tenantFromCtx(r.Context())
+	stats, err := s.store.StatsByTenant(r.Context(), tenantID)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to read storage stats")
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]interface{}{
+		"total_mails": stats["count"],
+		"total_bytes": stats["total_size"],
+	})
+}