feat: redirect / to the web UI (#101)

## Why

The web UI ships as a separate image served under \`/ui\` (built with \`BASE_PATH=/ui\`). Hitting the bare domain (e.g. \`https://artifactapi.k8s.syd1.au.unkin.net/\`) returned the API's JSON identity blob instead of the app, so browsers never landed on the UI.

## Changes

- Redirect \`GET /\` to \`/ui/\` (302 Found).
- Preserve the former root JSON (\`{"name","version"}\`) at \`/version\`, so health/monitoring can still read the running version.
- Update the server integration test to assert the redirect and the \`/version\` payload.

Reviewed-on: #101
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/server/server_test.go

@@ -129,13 +129,32 @@ func req(t *testing.T, method, path string, body string) (*http.Response, []byte
 	return resp, b
 }
 
+// reqNoRedirect issues a request without following redirects so the response's
+// status and Location header can be asserted directly.
+func reqNoRedirect(t *testing.T, method, path string) *http.Response {
+	t.Helper()
+	rq, _ := http.NewRequest(method, testTS.URL+path, nil)
+	client := &http.Client{CheckRedirect: func(*http.Request, []*http.Request) error {
+		return http.ErrUseLastResponse
+	}}
+	resp, err := client.Do(rq)
+	if err != nil {
+		t.Fatalf("%s %s: %v", method, path, err)
+	}
+	resp.Body.Close()
+	return resp
+}
+
 func TestServerHealthAndRoot(t *testing.T) {
 	requireStack(t)
 	if resp, _ := req(t, "GET", "/health", ""); resp.StatusCode != 200 {
 		t.Errorf("health: %d", resp.StatusCode)
 	}
-	if resp, b := req(t, "GET", "/", ""); resp.StatusCode != 200 || !strings.Contains(string(b), "test-version") {
-		t.Errorf("root: %d %s", resp.StatusCode, b)
+	if resp := reqNoRedirect(t, "GET", "/"); resp.StatusCode != http.StatusFound || resp.Header.Get("Location") != "/ui/" {
+		t.Errorf("root redirect: %d %q", resp.StatusCode, resp.Header.Get("Location"))
+	}
+	if resp, b := req(t, "GET", "/version", ""); resp.StatusCode != 200 || !strings.Contains(string(b), "test-version") {
+		t.Errorf("version: %d %s", resp.StatusCode, b)
 	}
 	if resp, _ := req(t, "GET", "/api/v2/health", ""); resp.StatusCode != 200 {
 		t.Errorf("health v2: %d", resp.StatusCode)