[good first issue] Add filter by date range to timeline API

[Carlos-Projects]

Task

Add start_date and end_date query parameters to the timeline API endpoint.

What to do

• Add optional start_date and end_date params to GET /api/timeline
• Filter events within the date range in TimelineBuilder
• Add tests for date filtering
• Update API documentation

Acceptance criteria

• GET /api/timeline?start_date=2025-01-01&end_date=2025-06-01 works
• All existing tests pass (python -m pytest tests/ -v)
• Lint passes (ruff check src/ tests/)

See CONTRIBUTING.md.