build: fix typescript errors

Author: chertik77

app/app.ts

@@ -33,7 +33,7 @@ appRouter.use('/board', boardRouter)
 appRouter.use('/column', columnRouter)
 appRouter.use('/card', cardRouter)
 
-app.use(process.env.API_PREFIX, appRouter)
+app.use(process.env.API_PREFIX!, appRouter)
 
 app.use((_, res) => {
   res.status(404).json({ message: 'Not found' })