fix(workers): reject events with non-object payload to avoid corrupted DB records

[Dobrunia]

Problem

Events with invalid payload types (e.g. payload: true instead of an object) were being stored in MongoDB. This led to:

• GraphQL error: Cannot return null for non-nullable field EventPayload.title
• Frontend error: TypeError: can't access property "event", l is null when loading project overview

Root cause: clients using beforeSend incorrectly (e.g. returning true instead of the event object) could send payloads that workers accepted and the grouper persisted as-is.

Solution

Add payload validation in workers so invalid payloads are rejected before processing:

• JavaScript worker: reject events where payload is not a plain object (null, array, boolean, string, number).
• Grouper worker: same check at the start of handle() so no invalid events are saved to events:* collections.

Invalid events are logged and dropped; they are no longer written to the DB.

Related PRs:

• fix(catchers): validate beforeSend return value to avoid sending invalid payload hawk.javascript#150 — client-side beforeSend validation
• fix(catchers): validate beforeSend return value to avoid sending invalid payload hawk.php#73 — client-side beforeSend validation
• fix(catchers): validate beforeSend return value to avoid sending invalid payload hawk.nodejs#46 — client-side beforeSend validation
• fix(workers): reject events with non-object payload to avoid corrupted DB records #515 — server-side hard validation (this PR)