feat: persist volume, play mode and audio quality across restarts

Runtime playback settings were kept only in memory (AudioPlayer.volume,
PlayQueue.mode, each provider's quality field), so restarting the bot reset
them to defaults and users had to re-tune volume and quality every time (#125).

Persist and restore them via the repo's existing storage:
- Volume and play mode are per-bot, stored on new bot_instances columns
  (volume, play_mode) with a schema migration; restored when the instance is
  (re)built, written by cmdVol / cmdMode which every entry point (chat command,
  WebUI, REST) funnels through. Volume and mode are written independently so a
  transient !fm/!artist mode switch never overwrites the user's saved !mode.
- Per-provider audio quality is global (shared providers), stored in a new
  config.json `audioQuality` block; applied to the providers at startup and
  re-snapshotted on POST /api/music/quality.

Queue, current song, progress and FM/artist sessions stay ephemeral.

Adds tests for config sanitize/round-trip, DB player-settings + migration,
cmdVol/cmdMode persistence + construction-time restore, and quality persistence
through the REST endpoint. Documents the behavior in the README.

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

README.md

@@ -414,6 +414,20 @@ sudo systemctl start tsmusicbot
 
 在设置页面选择音质，立即生效（影响后续播放的歌曲）。
 
+> **重启后保留（#125）**：音质选择会持久化到 `data/config.json`（每个平台各自记录），重启机器人后自动恢复，无需每次手动重设。
+
+### 重启后保留的播放设置
+
+以下运行时设置在改动时自动落盘，重启机器人后自动恢复，不再回到默认值：
+
+| 设置 | 作用范围 | 存储位置 |
+|------|----------|----------|
+| **播放音量**（`!vol` / WebUI 音量条 / REST `/volume`） | 每个机器人独立 | 数据库 `bot_instances.volume` |
+| **播放模式**（`!mode` / WebUI / REST `/mode`：顺序 / 列表循环 / 随机 / 随机循环） | 每个机器人独立 | 数据库 `bot_instances.play_mode` |
+| **音质**（各平台，WebUI 设置页 / REST `/quality`） | 全局（各平台各自记录） | `data/config.json` 的 `audioQuality` |
+
+聊天命令、WebUI、REST API 三种入口的改动都会被持久化。播放队列、当前歌曲、进度、`!fm` / `!artist` 等临时播放状态仍为一次性状态，重启后不保留（`!fm` / `!artist` 内部临时切换的随机 / 循环也**不会**覆盖你用 `!mode` 显式保存的偏好）。
+
 ## 项目架构
 
 ```