Add music bot for self-hosted Stoat with web control panel

Plays audio into Stoat voice channels over LiveKit and exposes the same
player through both chat commands and a browser panel, so the two never
drift apart: everything routes through a single MusicManager.

- core: per-server GuildPlayer (queue, loop, shuffle, seek, volume,
  idle auto-leave) driving revoice.js/@livekit/rtc-node and ffmpeg
- sources: yt-dlp for YouTube/SoundCloud, direct media URLs and internet
  radio, optional local library with path-traversal guards
- bot: 18 chat commands with aliases, plus !panel one-time login links
- api: Fastify REST + WebSocket, sessions authenticated against the
  instance's own /auth/session/login (TOTP supported), permissions
  re-checked against Stoat membership and roles on every request
- web: React panel with search, queue editing, seek and volume
- deploy: Dockerfile, compose.override.yml and Caddyfile snippets for
  dropping the service into an existing /opt/stoat stack

Verified with npm run typecheck, both builds, and scripts/smoke-api.mjs
(9 API checks). Voice playback itself needs a live instance to test.

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

deploy/compose.override.yml.example

@@ -0,0 +1,28 @@
+# /opt/stoat/compose.override.yml
+#
+# Раскладка:
+#   /opt/stoat       — ваш self-hosted Stoat (compose.yml, Caddyfile, ...)
+#   /opt/stoat-mbot  — этот репозиторий
+#
+# Сервис живёт в том же compose-проекте, поэтому попадает в сеть stoat_default:
+# Caddy видит его как http://mbot:3005, а бот — Stoat API как http://api:14702.
+services:
+  # ваш существующий override для Caddy — оставьте как есть
+  caddy:
+    ports: !override
+      - "127.0.0.1:8880:80"
+
+  mbot:
+    build: ../stoat-mbot
+    restart: always
+    env_file: ../stoat-mbot/.env
+    depends_on:
+      api:
+        condition: service_started
+    volumes:
+      # cookies.txt для yt-dlp и/или локальная медиатека
+      - ../stoat-mbot/data:/data
+    # Панель отдаётся через Caddy. Порт наружу нужен, только если панель
+    # проксирует внешний nginx на хосте, минуя Caddy:
+    # ports:
+    #   - "127.0.0.1:3005:3005"