Surface Stoat's voice errors and stop leaving orphan connections

Joining a channel failed silently: revoice's join() runs an async
executor inside `new Promise`, so a rejected join_call never reaches
reject() — the promise hangs forever and the real error escapes as an
unhandled rejection. The client wrapper now latches API failures and
settles the join itself, translating Stoat's error codes (AlreadyConnected,
LiveKitUnavailable, UnknownNode, ...) into messages the chat can show.

A failed join also used to leave the connection object alive, which kept
the bot registered in the channel and made the next attempt fail with
AlreadyConnected; it is now destroyed on any failure. The LiveKit node
name is configurable via VOICE_NODE for instances that renamed it, and
the README documents how to clear a stuck voice state from Redis.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.env.example

@@ -13,6 +13,10 @@ COMMAND_PREFIX=!
 # Удалять сообщение с командой после её распознавания (нужно право ManageMessages).
 DELETE_COMMAND_MESSAGES=true
 
+# Имя LiveKit-ноды из Revolt.toml, секция [hosts.livekit].
+# В стандартном self-hosted это "worldwide".
+VOICE_NODE=worldwide
+
 # ------------------------------------------------------------- Веб-панель ---
 PORT=3005
 HOST=0.0.0.0