Implement ActivityEvent Data Model

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**
Create the ActivityEvent model for OWASP Pulse to store unified GitHub activity events such as pull requests, issues, commits, and releases.

The model should support efficient querying and deduplication.

Supported event types:

- PR opened / merged / closed
- Issue opened / closed
- Commit pushed
- Release published

Include fields for:

- id
- activity_type
- occurred_at
- actor
- repository
- source_object (or equivalent reference to PullRequest, Issue, Commit, Release)
- created_at
- updated_at

Add appropriate database indexes for efficient timeline queries.

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

- [x] Yes
- [ ] No