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/discordBotService/index.js

@@ -46,6 +46,9 @@ const {
   undeterUser,
   muteUser,
   unmuteUser,
+  listAudioGainBoostUsers,
+  grantAudioGainBoost,
+  revokeAudioGainBoost,
 } = require('../verificationService');
 const {
   attachDmMessage: attachPrivateAccessDmMessage,
@@ -250,6 +253,9 @@ const commandDependencies = {
   undeterUser,
   muteUser,
   unmuteUser,
+  listAudioGainBoostUsers,
+  grantAudioGainBoost,
+  revokeAudioGainBoost,
   sanitizeMentions,
   sendToChannel: channelIO.sendToChannel,
   isAdminUser,