Add make add-dependency and make tidy (closes #45)

No entrypoint could change yarn.lock or go.mod: script/bootstrap
installs with --frozen-lockfile, so adding a package meant running
yarn by hand. make add-dependency PACKAGE=<name>@<version> shims to
the new script/add-dependency: yarn add --dev, then yarn install
--frozen-lockfile. make tidy shims to the new script/tidy, go mod
tidy in backend/; a Go module is added by importing it, or moved by
editing its require line, then make tidy. script/bootstrap is
unchanged.

Model: opus-5-5

script/tidy

@@ -0,0 +1,13 @@
+#!/bin/sh
+# script/tidy: run go mod tidy in backend/, which adds the modules the
+# Go sources import, drops those they no longer do, and updates go.sum.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT/backend"
+    go mod tidy
+}
+
+main "$@"