feat(ai-moderation): reset offensive nickname instead of deleting message

When the ONLY violation is offensive_username (message content clean):
- Message is NOT deleted (nickname-only violation bypasses auto-delete)
- Member's server nickname is reset to default username via
  setNickname(null) (Discord shows the global username again)
- Action 'reset_nickname' logged to moderation_actions; cooldown
  10min per guild:user (LRU) so repeated messages by same member
  don't hammer the Discord PATCH
- Config: AUTO_NICKNAME_RESET_ENABLED / AUTO_NICKNAME_RESET_COOLDOWN_MS

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

@@ -615,6 +615,7 @@ export const pgModerationActionsTable = pgTable(
         "warn_user",
         "kick_user",
         "ban_user",
+        "reset_nickname",
       ],
     }).notNull(),
     reason: pgText("reason"),