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

@@ -20,6 +20,7 @@ export interface ModerationAction {
   created_at: number | null;
   executed_at: number | null;
   username: string | null;
+  server_name: string | null;
   content: string | null;
   // ── Explainability (structured verdict, surfaced read-only to public web) ──
   flags: string[] | null;
@@ -80,6 +81,7 @@ export interface CategoryAction {
   confidence: number | null;
   score: number | null;
   username: string | null;
+  server_name: string | null;
   content: string | null;
 }