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.

README.md

@@ -40,6 +40,7 @@ make docker
 ```bash
 make bootstrap  # Install all dependencies (idempotent)
 make setup      # Bootstrap + install git pre-commit hook
+make assets     # Fetch + verify third-party browser assets
 make fmt        # Format code (gofmt + goimports)
 make lint       # Run golangci-lint
 make test       # Run tests with race detection
@@ -247,6 +248,8 @@ them. We provide:
 - `script/setup` — make a fresh clone ready for development
   (bootstrap, then install-precommit)
 - `script/projectname` — output the project name ("webhooker")
+- `script/fetch-assets` — download the third-party browser assets into
+  `static/`, verifying each against its pinned sha256
 - `script/test` — run the test suite
 - `script/lint` — run golangci-lint
 - `script/fmt` — format all code (writes)
@@ -260,6 +263,27 @@ them. We provide:
 - `script/install-precommit` — install the git pre-commit hook that
   runs `script/precommit`
 
+## Third-party browser assets
+
+The web UI serves one third-party script, Alpine.js. It is **not** committed:
+a minified bundle in the tree is unreviewable, and `REPO_POLICIES.md` bars
+both committed build artifacts and unpinned external references.
+
+Instead `script/fetch-assets` downloads it from a pinned URL, checks the
+download against a hardcoded sha256, and installs it under `static/`. The
+sha256 of every installed asset is recorded in `static/vendor.sha256`, and
+`static/vendor_test.go` re-hashes the bytes `go:embed` put in the binary
+against that manifest — so the pin is enforced on what actually ships, not
+merely written down. Any mismatch fails the build.
+
+`make bootstrap` runs the fetch for local development, and the Dockerfile
+runs it in the build stage; `.gitignore` and `.dockerignore` keep the
+artifact out of both the repo and the build context.
+
+To move to a new version: update the version, URL, and tarball sha256 in
+`script/fetch-assets` and the asset sha256 in `static/vendor.sha256`, then
+run `make assets && make check`.
+
 ## Rationale
 
 Webhook integrations between services are inherently fragile. The