Release 0.3.1: network interception helpers, fix release-token secret name

- Bump package.json/lock to 0.3.1, move Unreleased CHANGELOG entries under it
- Fix release CI job: secrets.GITEA_TOKEN can never be set (Gitea rejects
  that literal secret name), so the release job has been unable to create
  a Gitea release since it was added. Renamed to RELEASE_TOKEN throughout
  and set the secret on this repo.
- Bump install-pin examples (README, docs/CONSUMER.md) to #v0.3.1

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@levkin/playkit",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "description": "Shared Playwright + API test kit — browser helpers, API client, logging, performance, Grafana/Prometheus metrics",
   "license": "MIT",
   "type": "module",