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

@@ -134,6 +134,13 @@
       "when": 1787820000000,
       "tag": "0018_add_server_name_to_moderation_actions",
       "breakpoints": true
+    },
+    {
+      "idx": 19,
+      "version": "7",
+      "when": 1787830000000,
+      "tag": "0019_rename_server_name_to_server_nick",
+      "breakpoints": true
     }
   ]
 }