build: unify the gate so root make check covers the backend (closes #16)

Root make check, and with it the pre-commit hook, now gates the Go
backend too. The backend's Makefile targets are shims over
backend/script/*; script/cibuild builds both images and is the
workflow's only build step. Root make test runs both halves within one
30-second timeout.

Root make lint runs golangci-lint only in Docker, by building the lint
stage of Dockerfile.backend without the cache; the .golangci.yml drift
check moved into backend/script/lint. script/bootstrap installs no
linter: it reuses a Go at least as new as backend/go.mod asks for,
otherwise installs the pinned, hash-verified release, linked into
~/.local/bin without replacing anything it did not create. With VERSION
unset or empty, the backend version falls back to git describe inside a
git checkout, then to dev.

Model: opus-5-5

script/frontend-viewport-test

@@ -45,7 +45,7 @@ main() {
     cd "$ROOT"
 
     # Test what ships: the production build, not a dev server.
-    "$ROOT/script/test"
+    "$ROOT/script/frontend-test"
     if [ ! -f "$ROOT/dist/index.html" ]; then
         echo "frontend-viewport-test: dist/index.html missing after build" >&2
         exit 1