fix: capture server nickname (member displayName) per action

Rename server_name (guild name) to server_nick and populate it from
the member's server-specific display name (metadata.member.displayName)
at write time. This is what the moderation dashboard should show as
TARGET — e.g. server nick 'Bandar Togel「✔ ᵛᵉʳᶦᶠᶦᵉᵈ 」' for global
username '.nichiyobi'. Backfilled 210 existing actions from messages
metadata (reset_nickname rows now show 'Sarjana .jav', 'Penindas
Minoritas', etc). Frontend TARGET shows server nick with global
username as secondary context.

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

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