feat: add due date field to tasks

[ashudhanda]

Implements the due date feature requested in #4.

Changes

• POST /tasks now accepts a dueDate (ISO 8601 string) and rejects invalid values with a 400 error
• GET /tasks supports filtering with ?overdue=true (returns tasks whose dueDate is in the past and whose status is not done)
• PUT /tasks/:id can update dueDate (with the same validation; null clears the due date)

Notes

• Added an isValidDueDate helper using Date.parse for ISO 8601 validation
• Existing filters (status, priority, assignee) are unchanged and compose with overdue

Closes #4