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/frontend-check` is the frontend half of the gate, exposed as
  the `check-frontend` 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
  `check-backend` target is the mirror of it.
- `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.

READMEs at the root and in `backend/` document every script, and
`TODO.md` records the change.

backend/script/lint

@@ -0,0 +1,46 @@
+#!/bin/sh
+# script/lint: run the Go linter over the backend.
+#
+# .golangci.yml is standardized org-wide and must never be edited here
+# (REPO_POLICIES.md). Its last silent drift replaced the v2 schema with
+# v1 keys, which left every threshold in the file inert while the build
+# stayed green. This script therefore asserts the file still matches the
+# pinned copy byte for byte before the linter runs. The check is a local
+# hash comparison: no network, no remote schema, nothing unpinned in the
+# build path.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# sha256 of the pinned backend/.golangci.yml.
+GOLANGCI_CONFIG_SHA256="33ba2bf7fe4a44779d09b0fb31d6daf03685f8dc9d2bc417f963d7aabb0d17dc"
+
+# sha256 <file>: print the file's sha256, coreutils or Darwin/busybox.
+sha256() {
+    if command -v sha256sum >/dev/null 2>&1; then
+        sha256sum "$1" | cut -d' ' -f1
+    else
+        shasum -a 256 "$1" | cut -d' ' -f1
+    fi
+}
+
+check_config_hash() {
+    actual="$(sha256 .golangci.yml)"
+    if [ "$actual" != "$GOLANGCI_CONFIG_SHA256" ]; then
+        echo ".golangci.yml has drifted from the pinned config."
+        echo "  expected $GOLANGCI_CONFIG_SHA256"
+        echo "  actual   $actual"
+        echo "Restore it verbatim from sneak/prompts; do not edit it."
+        echo "Only update GOLANGCI_CONFIG_SHA256 in this script when the"
+        echo "pinned config is deliberately replaced with a new standard."
+        exit 1
+    fi
+}
+
+main() {
+    cd "$ROOT"
+    check_config_hash
+    golangci-lint run ./...
+}
+
+main "$@"