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>

src/index.ts

@@ -100,6 +100,15 @@ async function main() {
   if (jellyfinAuth) jellyfinProvider.setCookie(jellyfinAuth);
   jellyfinProvider.setPersist((serialized) => cookieStore.save("jellyfin", serialized));
 
+  // Restore the persisted per-provider audio quality (#125) onto the shared,
+  // process-wide providers so a restart keeps the user's choice. setQuality()
+  // normalizes/ignores unknown values, so a stale entry can never break playback.
+  neteaseProvider.setQuality(config.audioQuality.netease);
+  qqProvider.setQuality(config.audioQuality.qq);
+  bilibiliProvider.setQuality(config.audioQuality.bilibili);
+  kugouProvider.setQuality(config.audioQuality.kugou);
+  jellyfinProvider.setQuality(config.audioQuality.jellyfin);
+
   const permissions = createPermissionStore(db.db);
 
   // Single process-wide Spotify authorization (one Premium account for Stage 3).