Implement Periodic Sync

Community & Support: [LinkedIn Group](https://www.linkedin.com/groups/14656108/) · [Slack #project-nest](https://owasp.slack.com/archives/project-nest)

Active project leaders: Arkadii Yakovets -- [GitHub](https://github.com/arkid15r/) · [LinkedIn](https://www.linkedin.com/in/arkid15r/) · [Slack](https://owasp.slack.com/team/U060W3NKLTF); Kate Golovanova -- [GitHub](https://github.com/kasya/) · [LinkedIn](https://www.linkedin.com/in/kate-golovanova/) · [Slack](https://owasp.slack.com/team/U07PWB1JZ6Z)

[Contributing](https://github.com/owasp/nest/blob/main/CONTRIBUTING.md) · [Code of Conduct](https://github.com/owasp/nest/blob/main/CODE_OF_CONDUCT.md) · [GSoC Mentors](https://github.com/owasp/nest/blob/main/MENTORS.md)

---

**Description**
Implement a periodic sync system for OWASP Pulse to ensure activity data is captured reliably.

This sync should act as a fallback for missed webhook events and support activity ingestion for repositories without webhook integration.

The system should periodically read recent GitHub activity and convert it into `ActivityEvent` records.

**Supported sources:**

- Pull Requests
- Issues
- Commits
- Releases

The sync should support incremental updates based on the last successful run and avoid duplicate event creation.
Support scheduled execution for automatic syncing.

**Are you going to work on implementing this?**

- [x] Yes
- [ ] No