refactor: 优化合约类型导出与导入方式

- 导出合约类型以支持类型安全的接口定义
- 更新类型导入以使用 Contract 而非直接导出的 contract 变量

apps/server/src/api/contract/index.ts

@@ -3,3 +3,5 @@ import * as todo from './todo'
 export const contract = {
   todo,
 }
+
+export type Contract = typeof contract

apps/server/src/api/types.ts

@@ -3,9 +3,8 @@ import type {
   InferContractRouterInputs,
   InferContractRouterOutputs,
 } from '@orpc/contract'
-import type { contract } from './contract'
+import type { Contract } from './contract'
 
-export type Contract = typeof contract
 export type RouterClient = ContractRouterClient<Contract>
 export type RouterInputs = InferContractRouterInputs<Contract>
 export type RouterOutputs = InferContractRouterOutputs<Contract>