Showcase winner role: assign a role for n days to the user with the most votes #128

Why

A temporary role gives the most-voted poster visible recognition.

Acceptance Criteria

• After results are announced, the bot assigns a configured role to the author of the post with the most votes.
• The role is removed after n days.
• Removal survives a bot restart.
• The role assignment and expiry logic is covered by unit tests.

Open Questions

• Which role is used, and how is it configured in src/env.ts?
• What happens if the previous winner still has the role?
• How are ties handled?
• What happens if the winner has left the server?

Out of Scope

• The voting and the result message (separate child issue).