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/drizzle/migrations/0018_add_server_name_to_moderation_actions.sql

@@ -0,0 +1 @@
+ALTER TABLE "moderation_actions" ADD COLUMN "server_name" text;

services/discord-gateway/drizzle/migrations/meta/_journal.json

@@ -127,6 +127,13 @@
       "when": 1787810000000,
       "tag": "0017_add_username_to_moderation_actions",
       "breakpoints": true
+    },
+    {
+      "idx": 18,
+      "version": "7",
+      "when": 1787820000000,
+      "tag": "0018_add_server_name_to_moderation_actions",
+      "breakpoints": true
     }
   ]
 }