P2R-02 slice 2 follow-up: refuse to delete a host's last source group

Belt-and-braces: the UI now disables the Delete button when a group
is the only one on the host (with a tooltip explaining why), and the
server-side handler returns 409 if a curl/form-replay tries anyway.
Every host needs at least one source group to be backup-able, so the
'last group on a fresh host' case is a meaningful accident to guard
against.

internal/server/http/ui_sources.go

@@ -310,6 +310,20 @@ func (s *Server) handleUISourceGroupDelete(w stdhttp.ResponseWriter, r *stdhttp.
 		return
 	}
 
+	// Refuse to delete the host's last source group — every host
+	// needs at least one to be backup-able. UI disables the button
+	// in this case; this guards against form-replay / curl.
+	groups, err := s.deps.Store.ListSourceGroupsByHost(r.Context(), host.ID)
+	if err != nil {
+		slog.Error("ui sources: count groups", "err", err)
+		stdhttp.Error(w, "internal", stdhttp.StatusInternalServerError)
+		return
+	}
+	if len(groups) <= 1 {
+		stdhttp.Error(w, "this is the host's only source group — create another one first", stdhttp.StatusConflict)
+		return
+	}
+
 	if err := s.deps.Store.DeleteSourceGroup(r.Context(), host.ID, gid); err != nil {
 		if errors.Is(err, store.ErrNotFound) {
 			stdhttp.NotFound(w, r)