feat: add due date support to tasks

## Summary

Tasks need a `dueDate` field (ISO 8601 string) so teams can track deadlines.

## Motivation

Right now, tasks only have a `createdAt` timestamp. Teams need to set deadlines and know which tasks are overdue.

## Proposed Behavior

- `POST /tasks` accepts an optional `dueDate` field (ISO 8601 string)
- `PUT /tasks/:id` can update `dueDate`
- `GET /tasks?overdue=true` returns tasks where `dueDate` is in the past and `completed` is `false`
- A new utility function `getOverdueTasks(tasks)` handles the filtering logic

## Acceptance Criteria

- [ ] `POST /tasks` accepts an optional `dueDate` field
- [ ] `PUT /tasks/:id` can update `dueDate`
- [ ] `GET /tasks?overdue=true` returns only incomplete, past-due tasks
- [ ] New `getOverdueTasks(tasks)` utility function with unit tests
- [ ] All existing tests continue to pass

Start by locating the task handlers for POST /tasks, PUT /tasks/:id, and GET /tasks?overdue=true, then find the existing task tests and test command. Trace how task fields are accepted and filtered before adding due-date coverage. Done means all listed acceptance criteria pass, including unit tests for getOverdueTasks and the existing test suite.

Written by the indexing model from the issue text.