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/entities/level.entity.ts

@@ -0,0 +1,37 @@
+import {
+  Entity,
+  PrimaryColumn,
+  Column,
+  CreateDateColumn,
+  UpdateDateColumn,
+} from 'typeorm';
+
+@Entity('levels')
+export class Level {
+  @PrimaryColumn({ type: 'varchar', length: 191 })
+  id: string;
+
+  @Column({ type: 'varchar', length: 191, name: 'image_url' })
+  imageUrl: string;
+
+  @Column({ type: 'varchar', length: 191 })
+  answer: string;
+
+  @Column({ type: 'varchar', length: 191, nullable: true })
+  hint1: string | null;
+
+  @Column({ type: 'varchar', length: 191, nullable: true })
+  hint2: string | null;
+
+  @Column({ type: 'varchar', length: 191, nullable: true })
+  hint3: string | null;
+
+  @Column({ type: 'int', name: 'sort_order', default: 0 })
+  sortOrder: number;
+
+  @CreateDateColumn({ name: 'created_at' })
+  createdAt: Date;
+
+  @UpdateDateColumn({ name: 'updated_at' })
+  updatedAt: Date;
+}