Backend: POST /api/timelines to create a timeline

[lbsong01]

Implement the backend pieces required to create a new timeline via the API.

Tasks

• Application: CreateTimelineCommand(string Name, string Description) returning the new Guid.
• Application: CreateTimelineCommandHandler (MediatR) persisting via ITimelineRepository.
• Application: CreateTimelineCommandValidator (FluentValidation) — Name required, max 200; Description max 2000.
• Domain: confirm TimelineDefinition supports creation with validation.
• Infrastructure: add AddAsync + SaveChangesAsync on ITimelineRepository + EF implementation.
• API: add POST /api/timelines endpoint in TimelineEndpoints.cs returning 201 Created with Location header.
• API: wire FluentValidation so invalid input returns 400 problem details.

Tracked in TODO.md.