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/shared/database/schema.ts

@@ -643,6 +643,7 @@ export const pgModerationActionsTable = pgTable(
     evidence: pgText("evidence"), // JSON array of short quoted snippets
     policy_version: pgText("policy_version"), // rules.ts policy version string
     username: pgText("username"), // denormalized from messages table for retention safety
+    server_name: pgText("server_name"), // denormalized guild name for retention safety
   },
   (table) => ({
     messageIdIdx: pgIndex("idx_moderation_actions_message_id").on(

services/discord-gateway/src/shared/database/schema/messages.ts

@@ -60,6 +60,7 @@ export const pgModerationActionsTable = pgTable(
     evidence: pgText("evidence"), // JSON array of short quoted snippets
     policy_version: pgText("policy_version"), // rules.ts policy version string
     username: pgText("username"), // denormalized from messages table for retention safety
+    server_name: pgText("server_name"), // denormalized guild name for retention safety
   },
   (table) => ({
     messageIdIdx: pgIndex("idx_moderation_actions_message_id").on(