feat(commands): add 'rs gain' to grant the VIP audio gain boost

Admins can now run 'rs gain list|grant <vip>|revoke <vip>' from web chat or
Discord. Grant matches only against the verified list and revoke only
against current holders, so a nickname shared with an unverified visitor
reports not-found rather than resolving to someone ineligible. The action
joins the moderation set so lockdown narrows it to lockdown admins.

Also extracts the ceiling math into audioLevelsService/gainMath.js and
covers both it and the command with node:test suites.

Co-Authored-By: Claude <noreply@anthropic.com>

server/src/services/chatService/textCommands.js

@@ -22,6 +22,9 @@ const {
   undeterUser,
   muteUser,
   unmuteUser,
+  listAudioGainBoostUsers,
+  grantAudioGainBoost,
+  revokeAudioGainBoost,
 } = require('../verificationService');
 const { publishEvent } = require('../eventBus');
 const assignmentService = require('../assignmentService');
@@ -184,6 +187,9 @@ async function runChatTextCommand({ text, socket, sendSystemMessage }) {
     undeterUser,
     muteUser,
     unmuteUser,
+    listAudioGainBoostUsers,
+    grantAudioGainBoost,
+    revokeAudioGainBoost,
     sanitizeMentions,
     sendToChannel: null,
     isAdminUser: (id) => String(id) === String(socket.id) && isAdmin(socket),