✨ Activity feed — recent commits, PR merges, issue closures

## Description
A chronological activity feed showing recent developer activity across all tracked repos.

## Events to include
- Commits pushed
- PRs merged
- Issues opened/closed
- Releases published

## Acceptance Criteria
- [ ] `/api/activity` endpoint
- [ ] Timeline-style UI with event type icons
- [ ] Filterable by repo or event type
- [ ] Refreshes on poll cycle