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/cibuild

@@ -1,13 +1,17 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The Dockerfile runs make check, so
-# a successful build implies all checks pass.
+# script/cibuild: run the CI build. It builds both images: the frontend
+# from Dockerfile and the backend from Dockerfile.backend. Each runs its
+# half of the checks as build steps, so a successful cibuild implies the
+# whole repo is green. This is the only build step the Gitea workflow
+# runs.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build .
+    timeout 300 docker build .
+    timeout 300 docker build -f Dockerfile.backend .
 }
 
 main "$@"