fix(config): store config.json under the persisted data dir (#86)

CONFIG_PATH resolved to ROOT_DIR/config.json (/app/config.json in Docker), but only
DATA_DIR (/app/data) is the mounted volume — every other artifact (DB, cookies, logs,
avatars) already lives under DATA_DIR. So on first run the default config was written
into the ephemeral image layer (never appearing in the volume), and a manually-placed
data/config.json was ignored because the bot read/wrote the root path.

- Move CONFIG_PATH to DATA_DIR/config.json so it lands in the volume and manual edits
  take effect.
- Add migrateLegacyConfig(): one-time move of an existing root-level config.json into
  the data dir, so existing local installs keep their settings (no silent reset).
- Tests for first-run persistence + the three migration cases.
- README directory tree updated to data/config.json.

Fixes #86

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -383,11 +383,11 @@ teamspeak-music-bot/
 │   └── docker/                 # Docker 部署文件
 │       ├── Dockerfile
 │       └── docker-compose.yml
-├── data/                       # 运行时数据（自动创建，不上传）
-│   ├── tsmusicbot.db           # SQLite 数据库
-│   ├── cookies/                # 登录 Cookie
-│   └── logs/                   # 日志文件
-└── config.json                 # 配置文件（首次运行自动生成，不上传）
+└── data/                       # 运行时数据（自动创建，不上传）
+    ├── config.json             # 配置文件（首次运行自动生成，可手动编辑）
+    ├── tsmusicbot.db           # SQLite 数据库
+    ├── cookies/                # 登录 Cookie
+    └── logs/                   # 日志文件
 ```
 
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