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

Root `make check` only ever ran the frontend, so the "main is always
green" policy was satisfied vacuously: the Go backend could be entirely
broken and the root gate stayed green.

- The backend moves onto scripts-to-rule-them-all. Its test, lint, fmt,
  fmt-check, build, run and clean implementations now live in
  `backend/script/`, and `backend/Makefile` is thin shims. The backend
  is its own project (own module, README, LICENSE, linter config,
  Dockerfile stage), and `Dockerfile.backend` only copies `backend/`
  into its builder, so its scripts have to live under `backend/`.
- The root `script/test`, `script/lint`, `script/fmt` and
  `script/fmt-check` now run the frontend step and then the matching
  `backend/script/*` step, so `script/check` — and therefore the
  pre-commit hook — gates both halves. The frontend-only steps moved
  into `script/frontend-*` so nothing is duplicated.
- `script/bootstrap` now provisions the backend's toolchain as well,
  because widening the gate without widening bootstrap left the
  documented fresh-clone path (`make setup`) installing a pre-commit
  hook that rejected every commit with `golangci-lint: not found`. Go
  is reused when the installed version is new enough, otherwise it is
  installed from the official `go1.25.7` release archive; golangci-lint
  is installed at exactly `2.7.2`, the version `Dockerfile.backend`
  pins, so local findings match CI. Both come from a specific release
  archive whose sha256 is hardcoded here and verified before anything
  is unpacked — never an install script piped to a shell — and both are
  symlinked onto `PATH`, since nvm-style activation does not reach
  `make` or the git hook.
- `script/frontend-check` is the frontend half of the gate, exposed as
  the `frontend-check` target, for the frontend Dockerfile: its build
  stage is a node image with no Go toolchain. The backend half is gated
  by `Dockerfile.backend`, and `script/cibuild` builds both images, so
  the two Dockerfiles together still gate the whole repo. The
  `backend-check` target is the mirror of it. Both targets are named
  after the script they shim, like every other target.
- `script/cibuild` builds both images through one `build_image` helper,
  and the Gitea workflow's only build step is `script/cibuild`; the raw
  `docker build -f Dockerfile.backend .` is gone from the workflow.
  `script/docker` likewise builds and tags both images.
- `backend/Makefile`'s `hooks` target is removed. It wrote the same
  `.git/hooks/pre-commit` as `script/install-precommit`, so the two
  clobbered each other and the developer silently ended up gating on
  only one half of the repo. `script/install-precommit` is now the only
  installer, and the hook it writes runs the repo-wide `script/check`.
- `backend/Makefile`'s `docker` target is removed too: the backend image
  builds from the repo root with a root-level Dockerfile, so it belongs
  to the root `script/docker` and `script/cibuild` rather than to a
  backend script that would have to reach outside `backend/`.
- `backend/script/lint` verifies that `.golangci.yml` still matches its
  pinned sha256 before running the linter. Offline hash comparison, no
  network. The pin is marked provisional in the file: it is the config
  currently on `main`, and the comment names PR #31 and the canonical
  hash that must replace it when #31 lands.
- Every script locates the repo root with the mandated
  `$(cd "$(dirname "$0")/.." && pwd -P)` idiom, `cd`s there, and calls
  siblings as `"$ROOT/script/<name>"`; the `SCRIPT_DIR` variant is gone.

READMEs at the root and in `backend/` document every script, the
backend's Getting Started separates commands run from `backend/` from
those run at the repo root, and `TODO.md` records the change.

backend/Makefile

@@ -1,53 +1,38 @@
-UNAME_S := $(shell uname -s)
-VERSION := $(shell git describe --always --dirty)
-BUILDARCH := $(shell uname -m)
-BINARY := netwatch-server
+# Standard targets are thin shims; the implementations live in
+# backend/script/ per the scripts-to-rule-them-all pattern (see the
+# Entrypoints section of README.md).
+#
+# There is no `hooks` target here: the repo has exactly one pre-commit
+# hook installer, the root `script/install-precommit`, and the hook it
+# installs gates both halves of the repo. There is no `docker` target
+# either: the backend image is built from Dockerfile.backend with the
+# repo root as its context, so it belongs to the root `make docker` and
+# `script/cibuild`.
 
-GOLDFLAGS += -X main.Version=$(VERSION)
-GOLDFLAGS += -X main.Buildarch=$(BUILDARCH)
-
-ifeq ($(UNAME_S),Darwin)
-	GOFLAGS := -ldflags "$(GOLDFLAGS)"
-else
-	GOFLAGS = -ldflags "-linkmode external -extldflags -static $(GOLDFLAGS)"
-endif
-
-.PHONY: all build test lint fmt fmt-check check docker hooks run clean
+.PHONY: all build test lint fmt fmt-check check run clean
 
 all: build
 
-build: ./$(BINARY)
-
-./$(BINARY): $(shell find . -name '*.go' -type f) go.mod go.sum
-	go build -o $@ $(GOFLAGS) ./cmd/netwatch-server/
+build:
+	@script/build
 
 test:
-	timeout 30 go test ./...
+	@script/test
 
 lint:
-	golangci-lint run ./...
+	@script/lint
 
 fmt:
-	go fmt ./...
+	@script/fmt
 
 fmt-check:
-	@test -z "$$(gofmt -l .)" || \
-		(echo "Files not formatted:"; gofmt -l .; exit 1)
+	@script/fmt-check
 
-check: test lint fmt-check
+check:
+	@script/check
 
-docker:
-	timeout 300 docker build -t netwatch-server -f ../Dockerfile.backend ..
-
-hooks:
-	@printf '#!/bin/sh\ncd backend && make check\n' > \
-		$$(git rev-parse --show-toplevel)/.git/hooks/pre-commit
-	@chmod +x \
-		$$(git rev-parse --show-toplevel)/.git/hooks/pre-commit
-	@echo "Pre-commit hook installed"
-
-run: build
-	./$(BINARY)
+run:
+	@script/run
 
 clean:
-	rm -f ./$(BINARY)
+	@script/clean