fix: Tenant-Isolation in handleSearch + requireMailAccess

- handleSearch: Ergebnisse nach tenant_id filtern (via email_refs)
- requireMailAccess: domain_admin darf Mails lesen, Tenant-Prüfung
  erfolgt bereits in handleGetMail via GetTenantForMail

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/server.go

@@ -542,6 +542,26 @@ func (s *Server) handleSearch(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Tenant isolation: filter results to only this tenant's emails.
+	tenantID := tenantFromCtx(r.Context())
+	if tenantID != nil && len(result.Hits) > 0 {
+		allowedIDs, idErr := s.store.GetAllIDsByTenant(r.Context(), tenantID)
+		if idErr == nil {
+			allowed := make(map[string]struct{}, len(allowedIDs))
+			for _, id := range allowedIDs {
+				allowed[id] = struct{}{}
+			}
+			filtered := result.Hits[:0]
+			for _, h := range result.Hits {
+				if _, ok := allowed[h.ID]; ok {
+					filtered = append(filtered, h)
+				}
+			}
+			result.Hits = filtered
+			result.Total = len(filtered)
+		}
+	}
+
 	sess := sessionFromCtx(r.Context())
 	s.audlog.Log(audit.Entry{
 		EventType: audit.EventSearch,
@@ -768,12 +788,16 @@ func remoteIP(r *http.Request) string {
 
 // ── Mail access middleware ────────────────────────────────────────────────
 
-// requireMailAccess blocks admin role (no mail access) and passes user/auditor through.
+// requireMailAccess checks that the caller may read mail content.
+// superadmin and domain_admin have read access (tenant-scoped via handleGetMail).
+// Auditor and user have access to their own mails.
+// The old "admin" role (now domain_admin) previously had no mail access — that
+// restriction is removed; domain_admin now needs to be able to read archived mails.
 func (s *Server) requireMailAccess(next http.HandlerFunc) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		sess := sessionFromCtx(r.Context())
-		if sess.Role == userstore.RoleAdmin {
-			writeError(w, http.StatusForbidden, "admins have no access to mail content")
+		if sess == nil {
+			writeError(w, http.StatusUnauthorized, "not authenticated")
 			return
 		}
 		next(w, r)