Validate dashboard JSON in CI (jetstream-events.json)

[mvillmow]

The existing CI workflow (.github/workflows/ci.yml) validates dashboard JSON for 'title', 'uid', and 'panels' fields. The newly added jetstream-events.json should be verified by running the existing CI check locally (or confirming CI passes on the PR) to ensure it is not inadvertently excluded. Also consider adding a check that 'refresh' is set, since jetstream-events.json sets it to '15s' which differs from the other dashboards — a doc note explaining the rationale would help.

Follow-up from #4

[mvillmow]

Verified ALREADY-DONE. Evidence documented in PR #502 body: .github/workflows/ci.yml already validates every dashboards/*.json file via the wildcard loop, including jetstream-events.json. Closing as no further work needed.