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/frontend/src/lib/types/moderation.ts

@@ -20,7 +20,7 @@ export interface ModerationAction {
   created_at: number | null;
   executed_at: number | null;
   username: string | null;
-  server_name: string | null;
+  server_nick: string | null;
   content: string | null;
   // ── Explainability (structured verdict, surfaced read-only to public web) ──
   flags: string[] | null;
@@ -81,7 +81,7 @@ export interface CategoryAction {
   confidence: number | null;
   score: number | null;
   username: string | null;
-  server_name: string | null;
+  server_nick: string | null;
   content: string | null;
 }