fix: add server_name to moderation_actions for full context retention

Denormalize guild name alongside username so the moderation dashboard
shows both TARGET and server even after message table purges.
Migration 0018. Frontend displays 'username · server_name' in TARGET.

services/discord-gateway/src/modules/command-handler/moderation.handler.ts

@@ -104,6 +104,7 @@ export class ModerationHandler {
           | "ban_user",
         reason: payload.reason ?? null,
         username: null,
+        server_name: null,
         executed_by: payload.executed_by ?? "command-handler",
         status: "executed",
         error: null,