[003] Team Sprint Board (MVP)

## Overview

A collaborative sprint board for small teams. No authentication—users provide a display name and immediately join a single shared board. Board data persists until manually cleared.

Build this as a Python backend service with no UI; expose the functionality below through an HTTP API.

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## Identity

- Users must provide a display name before accessing the board.

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## Board Structure

- Four fixed columns: **Backlog**, **In Progress**, **Review**, **Done**.
- Columns cannot be added, removed, renamed, or reordered.
- Each column contains a list of cards.

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## Cards

**Card Data**
- Title (required)
- Description (optional)
- Story Points (optional): blank, 1, 2, 3, 5, 8, 13
- Status: determined by which column the card is in

**Creating Cards**
- Cards are created in a specific column with title, description, and story points fields.
- Title is required; submission with empty title returns a validation error.
- A card can be created with only title set.
- New cards appear in the column where they were created.

**Editing Cards**
- A card's title, description, story points, and status are editable.
- Changing status moves the card to the corresponding column immediately.
- A card can be permanently deleted.

**Moving Cards**
- Cards can be moved between columns by changing their status.

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## Clear Done

- All cards in Done can be permanently deleted in one operation.
- Cards in other columns are unaffected.

Start by locating the Python service entry point and reading how HTTP routes and persistent board data are organized. Define the API around display-name access, fixed columns, card validation, card moves and deletion, including clearing Done. Done means the documented operations persist correctly and enforce the stated validation and column rules.

索引モデルが issue の本文から書いたものです。