feat: add metadata timestamp (CM-868)

[ulemons]

Summary

Add metadata.commit_timestamp to the Kafka activities topic payload to align with the structure of other
topics coming from Sequin.

Background

All topics from Sequin have this structure:

{
  "metadata": {
    "commit_timestamp": "2025-03-17T02:51:12.741208Z"
  },
  "record": { ... }
}

However, the activities topic is pushed directly by data-sink-worker (not via Sequin), so it was missing the
metadata field.

Changes:

Also on the activities message now we have the structure:

{
    .....,
    "metadata": {
      "commit_timestamp": "2025-03-17T02:51:12.741208Z"
    },
}

---

Note

Low Risk
Small, additive payload change (new metadata field) with minimal logic changes; main risk is downstream consumers that assume a fixed message schema.

Overview
Activity messages emitted from insertActivities now include a metadata.commit_timestamp field (ISO timestamp) before being sent to the activities queue/Kafka, and logs were updated to include the new timestamp when dispatching.

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