Fetch and verify Alpine at build time instead of committing it (closes #145)

static/js/alpine.min.js was a committed minified bundle, which
REPO_POLICIES forbids, referenced by no content hash at all. A minified
blob is unreviewable, which is the shape a supply-chain compromise
takes.

script/fetch-assets now downloads Alpine 3.14.9 from the npm registry
and verifies sha256 on both the tarball and the extracted file, and
static/vendor_test.go re-hashes the bytes go:embed actually placed in
the binary. The shipped bytes are byte-identical to the blob that was
committed, so the served asset does not change.

Independently reviewed. Five negative controls reproduced by the
reviewer: flipped expected hash, repointed URL, post-fetch tampering,
asset absent, and manifest inconsistencies — each fails closed with
static/js/ left clean. Registry hashes confirmed against the pins, and
the runtime image was built, run and curled to confirm the asset is
still served and the login page still loads it.

Known gap, filed separately: static/static.go embeds the js directory
rather than named files, so a missing fetched asset is not a compile
error on ungated local build paths. Every gated path fails loudly, so
the release artifact is unaffected.

Dockerfile

@@ -32,7 +32,7 @@ FROM golang:1.26.1-bookworm@sha256:4465644228bc2857a954b092167e12aa59c006a349228
 # Depend on lint stage passing
 COPY --from=lint /src/go.sum /dev/null
 
-RUN apt-get update && apt-get install -y --no-install-recommends make && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y --no-install-recommends make curl ca-certificates && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /build
 
@@ -44,6 +44,14 @@ RUN go mod download
 # the lint stage above.
 COPY . .
 
+# Fetch the third-party browser assets the UI serves. They are not committed
+# (REPO_POLICIES.md forbids minified bundles in version control) and
+# .dockerignore keeps any host copy out of the build context, so this step is
+# the only way they enter the image. Each download is checked against a
+# hardcoded sha256 and the build fails on mismatch; make test re-checks the
+# hashes against the bytes go:embed actually put in the binary.
+RUN script/fetch-assets
+
 # Run tests and build
 RUN make test
 RUN make build