feat(discord): add Moderation and Event Handling prisms (#57)

[JamesJi79]

Discord Core Prisms - Moderation & Event Handling ($900)

Moderation Prisms ($500)

• BanMember — Ban with optional message deletion and reason
• UnbanMember — Remove ban
• TimeoutMember — Timeout with configurable duration
• ListBans — View banned users
• GetMember — Get member details

Event Handling Prisms ($400)

• CreateEvent — Create scheduled guild events
• ListEvents — List scheduled events
• DeleteEvent — Delete scheduled events

All prisms follow use Lux.Prism pattern with typed schemas, use existing Discord.Client, and return {:ok, result} | {:error, reason}.

Completes the remaining $900 scope for Issue #57 (Moderation + EventHandling).
MessageManagementPrism and ChannelManagement were already in the codebase.