Contribution Hub: Issue State & Deadline Reminders

We need to extend the issues functionality with deadline-based state and reminders.

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### Scope
- [ ] Show state of an issue based on its deadline date (e.g. overdue, due soon, completed). This should be shown on the issue list view (per row) and on the single issue view.
- [ ] Support sending a reminder to the assignee about the deadline. This should send some sort of a standard message to the issue, tagging assignee and requesting progress update.

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### ✅ Acceptance Criteria
- Each issue row in the issues list displays a state derived from the deadline.
- Single issue view also shows the state.
- Reminder functionality is available from the single issue page.

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**Are you going to work on implementing this?**

- [ ] Yes
- [x] No

Start with the issue list view and single issue view entry points, then trace how issue deadlines, assignees, and comments are represented. Define the deadline states and reminder behavior before implementation; done means both views show the derived state and the single issue page can send the specified assignee reminder.

Written by the indexing model from the issue text.