Add CRUD endpoints for game levels with level field in response:
- GET /v1/wechat-game/levels - list all levels ordered by sort_order
- GET /v1/wechat-game/levels/:id - get single level by ID
New files:
- Level entity mapping to levels table
- LevelRepository with ordered query support
- LevelResponseDto with level field (1-based index from sort_order)
