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/components/LiveModerationFeed.tsx

@@ -149,14 +149,15 @@ export function LiveModerationFeed({
                   <div className="mt-1.5 flex items-center gap-2 font-mono text-[10px] text-ink-faint">
                     <span>TARGET:</span>
                     <span className="text-ink-soft">
-                      {a.username ?? a.user_id ?? "UNKNOWN_SUBJECT"}
+                      {a.server_nick ??
+                        a.username ??
+                        a.user_id ??
+                        "UNKNOWN_SUBJECT"}
                     </span>
-                    {a.server_name && (
+                    {a.username && a.username !== a.server_nick && (
                       <>
                         <span>·</span>
-                        <span className="text-ink-soft">
-                          {a.server_name}
-                        </span>
+                        <span className="text-ink-soft">@{a.username}</span>
                       </>
                     )}
                   </div>

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;
 }