Based on the repository's commit message style and the changes in the diff, here's an appropriate commit message:

```
feat: add websocket tests, PoA metrics, marketplace endpoints, and enhanced observability

- Add comprehensive websocket tests for blocks and transactions streams including multi-subscriber and high-volume scenarios
- Extend PoA consensus with per-proposer block metrics and rotation tracking
- Add latest block interval gauge and RPC error spike alerting
- Enhance mock coordinator

apps/explorer-web/playwright.config.ts

@@ -0,0 +1,23 @@
+import { defineConfig, devices } from "@playwright/test";
+
+const PORT = process.env.EXPLORER_DEV_PORT ?? "5173";
+const HOST = process.env.EXPLORER_DEV_HOST ?? "127.0.0.1";
+
+export default defineConfig({
+  testDir: "./tests/e2e",
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 1 : 0,
+  reporter: process.env.CI ? "github" : "list",
+  use: {
+    baseURL: process.env.EXPLORER_BASE_URL ?? `http://${HOST}:${PORT}`,
+    trace: "on-first-retry",
+    viewport: { width: 1280, height: 720 },
+  },
+  projects: [
+    {
+      name: "chromium",
+      use: { ...devices["Desktop Chrome"] },
+    },
+  ],
+});