chore(阶段3): 删除阶段 1 的临时验证物

GET /api/dev/problems、dev-problems.vue、对应路由项，以及只服务于它的
problemSummarySchema。阶段 1 用来验证「能从真实库读出一道题」，已完成使命。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/web/src/oj/dev-problems.vue

@@ -1,31 +0,0 @@
-<script setup lang="ts">
-import type { ProblemSummary } from "@oj2/contract"
-import { onMounted, ref } from "vue"
-
-const problems = ref<ProblemSummary[]>([])
-const error = ref("")
-
-onMounted(async () => {
-  try {
-    const response = await fetch("/api2/problems")
-    const body = await response.json()
-    problems.value = body.data
-  } catch (cause) {
-    error.value = String(cause)
-  }
-})
-</script>
-
-<template>
-  <div style="padding: 24px">
-    <h2>阶段 1 链路验证（临时页，阶段 3 删除）</h2>
-    <p v-if="error" style="color: red">{{ error }}</p>
-    <p>共 {{ problems.length }} 道题</p>
-    <ul>
-      <li v-for="problem in problems" :key="problem.id">
-        {{ problem._id }} — {{ problem.title }}（{{ problem.difficulty }}，通过
-        {{ problem.acceptedNumber }}/{{ problem.submissionNumber }}）
-      </li>
-    </ul>
-  </div>
-</template>