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/package.json

@@ -5,10 +5,13 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test:e2e": "playwright test"
   },
   "dependencies": {},
   "devDependencies": {
+    "@playwright/test": "^1.48.0",
+    "@types/node": "^20.12.7",
     "typescript": "^5.4.0",
     "vite": "^5.2.0"
   }