feat: soundboard (room-scoped custom audio pads)

Server side of the soundboard feature:
- rooms.soundboard_enabled flag + soundboard_sounds table (self-bootstraps
  at boot via SoundboardManager; migration 021 seeds up-front)
- SoundboardManager loads enabled sounds and persists the per-room flag
- SoundboardPlayEvent broadcasts the pressed pad to everyone in the room
- SoundboardSetEnabledEvent (owner/staff) toggles the room flag and
  pushes refreshed settings
- settings (flag + sound list) sent on room enter, alongside YouTube

Database Updates/Own_Database_RunFirst/021_soundboard.sql

@@ -0,0 +1,15 @@
+-- Soundboard
+-- The room flag column + sounds table are also created at boot by
+-- SoundboardManager (ALTER ... ADD COLUMN IF NOT EXISTS / CREATE TABLE IF NOT
+-- EXISTS), so applying this file is only needed to seed sounds up-front.
+
+ALTER TABLE `rooms` ADD COLUMN IF NOT EXISTS `soundboard_enabled` TINYINT(1) NOT NULL DEFAULT 0;
+
+CREATE TABLE IF NOT EXISTS `soundboard_sounds` (
+    `id` INT(11) NOT NULL AUTO_INCREMENT,
+    `name` VARCHAR(64) NOT NULL DEFAULT '',            -- pad label shown in the client
+    `url` VARCHAR(255) NOT NULL DEFAULT '',            -- audio url (uploaded via CMS, like custom badges)
+    `enabled` TINYINT(1) NOT NULL DEFAULT 1,
+    `sort_order` INT(11) NOT NULL DEFAULT 0,
+    PRIMARY KEY (`id`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;