feat(wechat-game): add level list API endpoints

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)

src/modules/wechat-game/repositories/level.repository.interface.ts

@@ -0,0 +1,7 @@
+import { Level } from '../entities/level.entity';
+
+export interface ILevelRepository {
+  findAll(): Promise<Level[]>;
+  findById(id: string): Promise<Level | null>;
+  findAllOrdered(): Promise<Level[]>;
+}