[Task]: Implement milestones

[luizhcastro]

Objective

Implement milestone management system in the backend, including creation, update, deletion, and association with tasks. This will enable tracking progress across multiple related tasks.

Scope

In scope:

• Create milestone entity with properties (name, description, start date, end date, status)
• CRUD endpoints for milestone management
• Associate tasks with milestones
• API to retrieve tasks grouped by milestone
• Milestone progress calculation based on completed tasks

Out of scope:

• Frontend UI implementation (separate issue)
• Milestone dependencies
• Gantt chart visualization

Acceptance criteria

• Milestone entity created with all required properties
• POST /api/milestones endpoint working
• GET /api/milestones endpoint returning list
• GET /api/milestones/:id endpoint working
• PUT /api/milestones/:id endpoint working
• DELETE /api/milestones/:id endpoint working
• Tasks can be associated with milestones via API
• GET /api/milestones/:id/tasks returns all tasks in milestone
• Milestone progress calculation implemented
• Unit tests covering all endpoints
• API documentation updated

Deadline (optional)

No response