Fetch Alpine.js at build time under a verified hash (closes #145)

static/js/alpine.min.js was a committed minified bundle: unreviewable,
referenced by no hash, and forbidden by REPO_POLICIES.md on both counts.

It is now fetched by script/fetch-assets from a pinned npm registry
tarball, with the tarball sha256 and the extracted file's sha256 both
hardcoded and checked before anything is installed. The hash of every
installed asset lives in static/vendor.sha256, and static/vendor_test.go
re-hashes the bytes go:embed actually put in the binary against that
manifest, so the pin is enforced on what ships rather than merely
recorded. .gitignore keeps the artifact out of the repo and
.dockerignore keeps a host copy out of the build context, so the image
can only get it by fetching and verifying it.

Alpine 3.14.9 is byte-identical to the blob that was committed
(3ed1eed252488921df65e363d6715deb04d7f92aaedb9e52199fdf73cb1e0ad3), so
the served asset does not change; internal/server/static_assets_test.go
fetches every /s/ script base.html loads through the real router to
prove the page still gets it.

script/fetch-assets

@@ -0,0 +1,104 @@
+#!/bin/sh
+# script/fetch-assets: download the third-party browser assets the web UI
+# ships and install them under static/. Minified bundles are not committed
+# (REPO_POLICIES.md: no build artifacts in version control), so the build
+# fetches them here. Every download is verified against a hardcoded sha256
+# before it is installed, and any mismatch aborts. Idempotent: an asset
+# already present with its pinned hash is left alone.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# The sha256 of each installed asset lives in static/vendor.sha256, in
+# sha256sum(1) format, with paths relative to static/. That file is the
+# single source of truth: this script verifies against it, and
+# static/vendor_test.go asserts the bytes embedded into the binary match
+# it, so the hash cannot rot into a value nothing checks.
+MANIFEST="static/vendor.sha256"
+
+# Alpine.js 3.14.9, 2026-08-17. Fetched from registry.npmjs.org, the
+# publisher of record; the jsDelivr and unpkg copies are mirrors of this
+# same tarball. dist/cdn.min.js is the browser build Alpine publishes for
+# a <script> tag.
+ALPINE_VERSION="3.14.9"
+ALPINE_URL="https://registry.npmjs.org/alpinejs/-/alpinejs-${ALPINE_VERSION}.tgz"
+# sha256 of alpinejs-3.14.9.tgz
+ALPINE_TARBALL_SHA256="97dad7c0c81e659cfc8e7700055da9770f8186187cb9a8a76efb57e00d5ce52a"
+ALPINE_MEMBER="package/dist/cdn.min.js"
+ALPINE_DEST="js/alpine.min.js"
+
+sha256_of() {
+    if command -v sha256sum >/dev/null 2>&1; then
+        sha256sum "$1" | cut -d' ' -f1
+    else
+        shasum -a 256 "$1" | cut -d' ' -f1
+    fi
+}
+
+# expected_sha256 <path-relative-to-static>
+expected_sha256() {
+    awk -v want="$1" '$2 == want { print $1; found = 1 }
+        END { if (!found) exit 1 }' "$ROOT/$MANIFEST"
+}
+
+# verify <file> <expected-sha256> <what>
+verify() {
+    actual="$(sha256_of "$1")"
+    if [ "$actual" != "$2" ]; then
+        echo "fetch-assets: sha256 mismatch for $3" >&2
+        echo "  expected: $2" >&2
+        echo "  actual:   $actual" >&2
+        exit 1
+    fi
+}
+
+# up_to_date <path-relative-to-static> <expected-sha256>
+up_to_date() {
+    [ -f "$ROOT/static/$1" ] || return 1
+    [ "$(sha256_of "$ROOT/static/$1")" = "$2" ]
+}
+
+fetch_alpine() {
+    want="$(expected_sha256 "$ALPINE_DEST")"
+
+    if up_to_date "$ALPINE_DEST" "$want"; then
+        echo "fetch-assets: static/$ALPINE_DEST already at $want"
+        return 0
+    fi
+
+    echo "fetch-assets: fetching Alpine.js $ALPINE_VERSION from $ALPINE_URL"
+    tmp="$(mktemp -d)"
+    trap 'rm -rf "$tmp"' EXIT INT TERM
+    curl -fsSL -o "$tmp/alpine.tgz" "$ALPINE_URL"
+    verify "$tmp/alpine.tgz" "$ALPINE_TARBALL_SHA256" "alpinejs-${ALPINE_VERSION}.tgz"
+    tar -xzOf "$tmp/alpine.tgz" "$ALPINE_MEMBER" >"$tmp/alpine.min.js"
+    verify "$tmp/alpine.min.js" "$want" "$ALPINE_MEMBER from alpinejs-${ALPINE_VERSION}.tgz"
+
+    mkdir -p "$(dirname "$ROOT/static/$ALPINE_DEST")"
+    cp "$tmp/alpine.min.js" "$ROOT/static/$ALPINE_DEST"
+    rm -rf "$tmp"
+    trap - EXIT INT TERM
+    echo "fetch-assets: installed static/$ALPINE_DEST ($want)"
+}
+
+# Re-check every manifest entry against what is now on disk, so an entry
+# no script installs fails loudly instead of passing silently.
+verify_manifest() {
+    while read -r want path; do
+        case "$want" in '' | '#'*) continue ;; esac
+        if [ ! -f "$ROOT/static/$path" ]; then
+            echo "fetch-assets: $MANIFEST lists static/$path, which is missing" >&2
+            exit 1
+        fi
+        verify "$ROOT/static/$path" "$want" "static/$path"
+    done <"$ROOT/$MANIFEST"
+}
+
+main() {
+    cd "$ROOT"
+    fetch_alpine
+    verify_manifest
+    echo "fetch-assets: all assets in $MANIFEST verified"
+}
+
+main "$@"