feat(阶段1): monorepo 骨架与 @oj2/contract 契约包

packages/contract/package.json

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+{
+  "name": "@oj2/contract",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "exports": { ".": "./src/index.ts" },
+  "dependencies": {
+    "zod": "^4.0.0"
+  }
+}

packages/contract/src/index.ts

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+export * from "./problem"

packages/contract/src/problem.ts

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+import { z } from "zod"
+
+/** 题目列表项。字段取自 problem 表，只含列表页需要的列。 */
+export const problemSummarySchema = z.object({
+  id: z.number().int(),
+  _id: z.string(), // 展示用编号，与自增 id 不同
+  title: z.string(),
+  difficulty: z.string(),
+  submissionNumber: z.number().int(),
+  acceptedNumber: z.number().int(),
+})
+
+export type ProblemSummary = z.infer<typeof problemSummarySchema>

packages/contract/tsconfig.json

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+{
+  "extends": "../../tsconfig.base.json",
+  "include": ["src"]
+}