Add blog schema with BlogPost entity and shop blog toggle.

Introduces blog_posts table and blogEnabled on shop_settings for optional storefront blog.

backend/src/modules/shopSettings/entities/ShopSettings.ts

@@ -32,6 +32,9 @@ export class ShopSettings {
     @Column({ type: 'integer', nullable: true })
     simplexNotificationContactId: number | null;
 
+    @Column({ type: 'boolean', default: false })
+    blogEnabled: boolean;
+
     @CreateDateColumn()
     createdAt: Date;