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/docker

@@ -1,6 +1,7 @@
 #!/bin/sh
-# script/docker: build the Docker image tagged with the project name.
-# The tag comes from script/projectname.
+# script/docker: build both Docker images, tagged with the project name
+# from script/projectname: the frontend as <name>, from Dockerfile, and
+# the backend as <name>-server, from Dockerfile.backend.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -8,7 +9,9 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    timeout 300 docker build -t "$("$SCRIPT_DIR/projectname")" .
+    name="$("$SCRIPT_DIR/projectname")"
+    timeout 300 docker build -t "$name" .
+    timeout 300 docker build -t "$name-server" -f Dockerfile.backend .
 }
 
 main "$@"